Concidering a game a classic shouldnt be a matter of age but of quality. Many of the games emulated by mame are quite old but also quite crap. The life of a console system or a videogame is far to short for the standard rules to apply. With cars there is a 20 year window until a vehicle is officially recognised as a classic, but there are many cars much older than that which arent worth anything. On the otherhand just try to find a Buick Grand National or a Diamond Edition TransAm both less than 10 years old and see what you will have to pay.
Desirability is what makes a classic when it comes to cars and the same applies to games.
I think a game becomes a classic when the game is out of production, and yet, people still are out wanting to buy it. The most recent example I can think of would be the Final Fantasy games on Gameboy. Sunsoft finally revived them, and I noticed that they are selling quite well. So if a game isn't made anymore, and people still want to play it, it is going to be a classic. That's a tough call.