I like the two generations back rule. So now that the Wii U is out Dreamcast/PS2/Gamecube/Xbox/GBA games can be considered. If you did only one gen back then Pandora's Tower could qualify as it is clearly "last gen", as would brand new Xbox 360 games. Handhelds can be effectively grouped with the consoles of their era (3DS with Wii U, DS with Wii, etc.) but you run into a problem with computer games as they don't have clear generations. You would have to go with just age on those (ten years seems appropriate). The same issue would apply to arcade games, though they tend to get grouped by their home console conversions.
To me a game is a classic if it still merits significant conversation in the present day, either because it innovated in a way that influenced games that came later or it is just so damn good that people still play it and use it as an example of its genre. What qualifies "significant conversation"? Well that's really a matter of opinion. You just have to observe gaming culture and see what comes up. Something like Super Metroid gets talked about so damn often that you would be nuts to not acknowledge that it is regarded to be a classic.
Though in the context of a tradeshow called something like "Classic Gaming Convention" or a store selling used "Classic Games" the term would be interchangable with "retro" as it would merely mean a game of a certain age.