Of course first of all, a classic can be anything so long as the owner wants to think so. My consideration for what a classic car is how relevant the car is to the segment it occupies, what it meant for the company that made it and how it's looked on today. I can also say that these requirements can both swing negative or positive. 
For instance, a car I'd consider a classic is the Aston Martin Lagonda. It's ugly as hell, (as far as I know) terribly built and contributed to Aston Martin's bad times in the 80's. It's also super quirky, relatively rare and by all accounts positively unique. I'd also look at things in a more positive light with for instance something like my own car, a '91 Skyline GT-R. It's arguably one of Nissan's most famous nameplates (maybe tied with Z's or the 510), has a legendary track presence and is still beloved by fan around the world.

lurklove I totally agree that a fanbase makes a classic
I was talking with a Bolivian friend, and it gets to the point it is even a little bit weird how much they love the Toyota Corolla (of ALL generations, not only the 86 variety). Even a AE111 is a "amazing car" out there. It has a huge fanbase, it is used by many people that are proud of it. In the rest of the world, perhaps, not the most valuable of them all. But in that place, man they certainly consider it a classic.
Isnt that intersting? I mean, all the things have the value people tag it. Global classics like a GTR are undoubitely classics, but for smaller audiences, you can have a wider array of options to count in as classics. I think it gets down to the fanbase...

lurklove Welp. First comment nailed it on the head. Discussion over. 
Haha. But seriously, i think you nailed it. I don't think age affects it being a classic in anyway. For instance, I already consider the ZN6 chassis a classic. It became the closest to a pure sports car in the modern day market that didn't destroy wallets. Tuning companies showed just what its capable of. And as we speak, the car keeps evolving. It's  definitely left its mark and, in my view, won't ever be forgotten. That's what makes it a classic.

Lurklove got pretty close to the mark for me, but I think a huge part of a Classic car is the motorsport history. Plenty of cars come and go without so much of a blip on the radar. The R32 GT-R is a classic car for me because of it's history in motorsport in Australia and around the world. Even looking at a more Occa Australian reference in the VK/L, while these will never be cars I seek out, they will always be Classics, but generations have come and gone since these models and these younger cars will never be classed as a classic.
I'm very interested to see how the 86/BRZ holds up to the classic title being based off a true classic. Because Holden Australia tried to do the same thing years ago with the Monaro (Americans read Pontiac GTO) and while it was an interesting concept it will never reach Classic Status for me.

The classic cars in Japan have to do with their legacy. Skyline,Rx-7 ect... won races against major manufacturers. It was against giants. Those dreams transcended into the working man. I can own the car that beat ferrari ect...  That is the Japan view. America was way different. Cars built to race by the factory. The muscle car era.

Age is what first comes to mind for most people when they think classic cars. Sure, age plays a large part in the definition of a classic car but age is only a part of it. 
Picture the newer generations who will grow up having cars like the FR-S as their first car that might have no idea of it being the successor of the 86 we all know and love. In twenty-five years (here in California at least) the car will have had made a name for itself going down in their history as the FIRST FR-S of whatever successors it may have. Now since they've been able to grow with the car they'll be the ones to define its classic status. To older generations the FR-S is as new a successor they'll ever get and in present time have no idea of whether or not it will be able to hold its own against the following of THE 86 that started it all. They already have an image in their mind of what the 86 meant to them so anything after that (FR-S) will be considered new; but, just because it's new to some doesn't mean it isn't a classic. I believe classic cars are created not in when they were produced but rather what they mean to certain people.

I heard a great quote once.
'History makes its choices bad cars disappear , those that are still around have originality and value'.

To me, for a car to be a "classic" it has to have a following for its rarity, quality, and ability to retain or gain value regardless of its age - new or old.

All cars have an expiration date. When lots of people choose to keep a particular car alive far past its date - that makes it a 'classic'

I'm being a bit facetious, but not completely... It has to be a well liked model of car and also a well-taken care of example of said model to be a classic, and must be older than the youngest people able to legally drive in one's part of the world.

A classic is not always viewed as a cool car when it is released. Many times only a handful of buyers know the car's real worth... The manufacturer then do not always produce enough of these cars as the initial demand is very low. When the car is no longer available and when the initial costs of the car depreciates only then do the rest of us take notice of the car's potential. The car instantly becomes a rare classic... Today it is difficult to miss a possible classic, as forums like this exposes what would otherwise be rare jewels...
How many people actually knew of the Nissan GTR outside Japan/Asia and Australia before the R32 was released?

Fearedisx lurklove You just reminded me of something now! Shen we start mentioning the chassis code of a particular car, then it instantly becomes a classic.
I mean, who tries to remember the chassis code of something like 'Lada' maybe... (I meant no offence to Lada owners)

That it looks fresh and interesting 20 years after it's production.

for me a classic is not just a car that is at least 20 years old. but a car that has either changed the automotive world, been ground braking in engineering or design, developed a fallowing, or had a great race history. for me a car must meet at least the age requirement of 30 years as well as 2 of the others categories. The 87 rx7 is not there yet, its a few years out but in a few years it will be there. it will meet my age requirement in about 3 years, the car was ground braking in engineering with its handling, it has a great race history and it has developed a fallowing with the drift community there other reasons as well but those were the first ones that came to mind.

Je pense que tout dépend de l'aura que la voiture dégage et aussi comment elle a pu marquer les esprits de chacun...

It's all about popularity, and the only measure of popularity is sales.  Ford Taurus, Toyota Camry, Hyundai Elantra, these are the best selling cars, and the only cars worthy of being called classics.
Or is it the alternatives to these bland transportation appliances that are the real classics?  The cars we seek out while everyone else is buying lemming-mobiles.

it is the fact that it is really good that makes it a classic. many german, japanese or english cars are considered classics and are sold for fortunes. however rare are the french cars that are classics simply because they never been good.

I think a classic is a car that is sought after when its old. It is something that brings back memories for some people or was groundbreaking or different at one point in its life. Its definitely not about just being old. Do you really think a Ford Tempo or a Hyundai Accent are going to be classics when they get old? Most likely not

For me three things define a truly classic car: fun to drive? If yes, then is it older than me (I'm twenty), if yes how desirable is it on the used car market, if more desirable than an EF Civic SI (which is highly desirable for me) than it's a classic car of some sort! 
I have a 1996 Montego Blue Miata. I don't consider it classic

For a car to become a classic it should create an emotional reaction (passion) on people, not necessarily related with performance. For instance the Citroen CV: is it fast? No. Does it handle? No. Is it fun? Hmm not sure..
Same thing for the VW bug, early Fiat 500, and so on.
Some people say the R35 does not have any soul and because of that some people say it will not become a classic per ce. However, that lack of soul/asceptic/digital experience is overpowered by its overall performance: acceleration, cornering, sound, smell and everything else

A classic car isn't determined by the automakers or the mere fact that a car is old. In my eyes, the cult following of enthusiats, notable events or even the fact that a car was produced in limited numbers...that's what makes a car a classic. it's something that gets defined over the years, by the people.

Classic cars must always have one feature. It's design must become synonymous with an era. It's performance/engineering the epitome of it's category. 
Cars like the S13 hatchback, AE86, TA22, KE70, 240Z would all be considered designs that defined the era they were from and their performance/fun unmatched behind the wheel. 
Muscle cars all adhere to this, super cars, etc. and depending on the category one aspect may be more important than the other. AE86 is boxy so not universally beautiful, but one of the best "drivers" cars built in the 80's and is still hard to beat today. Supercars need both in spades. Sedans like the 60's Impalas, 40's Cadillacs, are obviously more style oriented.
Some people speak of age, but I don't feel that is important at all. Cars like the Z32, JZA80, even the Z06 Corvette all have those classic elements.

A classic car should be a rare, wanted, and amazing looking car, something like a 1985 corolla GTS, to a 1995 SUPRA, I dont think they have to go beyond the 30 years, I just feel like my 95 SUPRA is just as appreciated as much as my uncles 69 CAMARO. Vehicles like the FD RX7, R34, R32, R33, NSX and  few more are already classics, but everyone thinks different. =)

What defines a classic car, for me, is love.  If the owner loves an old car, ANY old car, enough to keep it in a good state of repair (running, streetable), it is a classic.  As an older guy myself, I've been around long enough to know that today's "junker", unfashionable, cheap, or unregarded make/model can be tomorrow's superstar classic car (I've seen it happen to many marques & models).  So, it's not the demand or fashion which makes a car a classic.  It's the dedication of its owner who keeps it clean, keeps it running, keeps its presence and experience as a rolling piece of art alive that matters.  It's the love.

DavidWoolery Hit the nail on the head for me! As long as the owner loves it, and takes care of it, it's a classic.

In my opinion, a car turns classic when it is successfully do your job. Some cars are instantly classics when they are real good performing what they are made for

Classics should have legendary status, often founded on controversy, purpose, craftsmanship, vintage, mystery, appeal... not just mere sales figures and iconic bandwagoning. Vehicles made in limited productions, unique features, turn of the century styling, and also a great back story make for a great classic car.

For me, it's about the car makes you feel, but like everything, there is an associated logic! It doesn't matter if it has 50 or 500 HP. Until now, only older cars made me feel special, and that's why I think that this is the key to know when a car is a classic.
I believe in classics for everyone, like the F40, the E-type, the original mini, the Mustang, the 240Z or the 911 (until the 993); we don't need to drive them to know that. But I believe in simpler classic cars too, not for everyone but only for some people, a special car that you have or had in your life. I think you have two kind of classics, the logic ones and the sentimental ones. For some people, a classic car can be both, if you're the happy owner of a F40, for example!
The logic ones are classics because of the way their drivers feel, and more importantly because of their history (or maybe equally their drivers point of view). the sentimental ones are classics because of some history associated and, more importantly because of the way their drivers feel.
It's a mixture of history and feeling!...

For me, it's about the way the car makes you feel, but like everything,
there is an associated logic! It doesn't matter if it has 50 or 500 HP.
Until now, only older cars made me feel special, and that's why I think
that this is the key to know when a car is a classic.
I believe in
classics for everyone, like the F40, the E-type, the original mini, the
Mustang, the 240Z or the 911 (until the 993); we don't need to drive
them to know that. But I believe in simpler classic cars too, not for
everyone but only for some people, a special car that you have or had in
your life. I think you have two kind of classics, the logic ones and
the sentimental ones. For some people, a classic car can be both, if
you're the happy owner of a F40, for example!
The logic ones are
classics because of the way their drivers feel, and more importantly
because of their history (or maybe equally for their drivers).
the sentimental ones are classics because of some history associated
and, more importantly because of the way their drivers feel.It's a mixture of history and feeling!...

I think the answer is simple: It's a car that is cool long after its heyday. The key to that sentence being that "cool" is subjective and can be many different things to many different people. And to the subject of an "instant classic", that is a car that people anticipate will still be cool when it's past its prime.

a car becomes a classic when youre able to park your car and still look back at it in awe as youre walking away. its a feeling where no matter how old or how long youve driven it you still feel that same exact joy the first time you've encountered an experience with it- everyday.

I believe there are two kinds of classics. One kind of classic is a car that is an icon from the start, like the Ae86 and a future classic the S2000. You know... cars that are specials and can never be replaced. And the 2nd kind of classic are your own personal classics. For me my 1994 Geo Prizm will forever be a personal classic to me. That car holds a special place in my heart. Sure to other's it seems like a piece of junk but the memories that i've had in that car are priceless. One day it shall recieve an epic revival!

It's important to understand what a classic of any type is. A classic recipe, piece of clothing, or melody all share some similarities: they are old, they haven't changed much, they maintain a certain amount of desirability--although never the most at a given time, and probably most importantly, their desirability is fairly stable in the face of other changes. In that sense McDonald's is a classic restaurant. Classic-ness is a "measure" of desirability. Some comments have mentioned utility and fashion, and I think they're both ends of the same desirability continuum.
Many comments try to define the word classic, using cars as an example, but I think Mike Garrett may have been asking what qualities make a car desirable at all. Here're my ideas:
1) History: Bart_205 is right in saying history plays a role. The Porsche 550 Spyder has some classic cred because James Dean died in one. Personal history can make a car a personal classic, like the first car you owned, or the car you drove from the wedding chapel.
2) Aesthetic originality: Being different, bold, and daring makes you stand out, improving the chances of being remembered. You can't be a classic if you're not remembered. 
3) Innovation: But it's not just any memory. It's better to be remembered for the being the first to introduce an innovation that has become standard; (or at least be known for being an innovator, whether it's true or not). E.g.: Volvo is known for seat belts, Jaguar is known for disc brakes, Cord had the first cruise control. Chrysler had the first downdraft carburetor, but the brand lacks a certain classic cred. I submit that's because they weren't successful at linking their name to the innovation. Maybe they were back in the '20s and '30s, but we rarely use carbs anymore; so nobody cares. Chrysler's also produced a lot of duds, which has watered down their brand.
4) Exoticism: this is the magic some commentators wrote about. It comes from a sense of unattainability and "otherness." Read one of Dino Dalle Carbonare's articles (with all due respect to his excellent eye for material culture) and you get a sense of a white fanboy lusting over his Asian classmate's younger sister. That's exoticizing a person; but we do that with objects, like cars, too.
5) Relevance: Critics say Star Wars (Episode IV: A New Hope) became a classic because its themes struck a chord with the U.S. public and its mood at that particular time in history. If the movie were to come out now, it'd probably flop; kinda like episodes I to III, which only did as well as they did due to its history. The VW Bug became a classic in part because it fit into people's values systems (I value personal mobility) and improved their lives (hey, I can afford one!).
6) Continuity/consistency: Not changing sends a message we're good the way we are, regardless of objective quality standards. (Harley Davidsons are mechanically crap, but if you accept this quirk, you "get" the H-D ethos and tradition). This is probably the most important characteristic of being a classic, by my definition above. Classics are inherently conservative.

For me,is a machine what gives you more than emotions to drive than other contemporary cars

miksfield Maybe the Polo R is the closest to a homologation model... but this i do not think will be a classic

miksfield Maybe the Polo R is the closest to a homologation model... but this i do not think will be a classic

Have you seen this? Some guy in Virginia is restoring a 1952
Hudson Hornet convertible and installing hand controls so he can share the ride
with disabled folks.
Check out the story -
http://www.classicandsportscar.com/news/general-classic-car-news/hudson-hornet-to-be-rebuilt-for-disabled-drivers
There is a link at the bottom for folks who wish to
contribute! Cool!