toxicTom: In my experience most people regard Witcher 1 (2007) and TES 4: Oblivion (2006) as pretty old now.
The current "borderline" year of 2010 saw titles like Alan Wake, Bioshock 2, Darksiders, Fallout: New Vegas, Mass Effect 2, Metro 2033, Star Wars: The Force Unleashed II, Two Worlds II...
... interesting how some of them feel rather dated now, while others still feel very recent.
Was thinking after I posted that that Witcher 1 would be a counter-example to what I said, since I am starting to think of that as a classic by now. But then again, it was based on older tech, albeit heavily modified.
None of those you listed from 2010 strike me as classics. Neither does Oblivion, not in the least, in fact Oblivion is one I specifically think of as a notable representative of a shift to a new and much worse mindset in developing games.
timppu: It appears to me to many of these people "old game" means "a game from the era before Steam existed, and which can't be bought from Steam". So, if it is a 11 year old game that was released on Steam already, nope, that is not an "old game" that GOG should sell. Anything that was released during the time Steam has been a thing, is "too new", from here to eternity.
Hmm, game released before Steam existed... That does seem like a decent separator. But it does move things about a year earlier, and isn't relevant in itself. (Not being sold on Steam is just silly as a condition though.)
Does tie in with something else though. The release of the Source engine, so HL2 but also, again, Bloodlines. Can take that as a moment that made tech maybe more important than the ideas and dedication of the devs, and when you connect it with the fact that it's Valve's tech and they ended up deciding the sale model for gaming too, seems even more fitting as a separator between classic and "modern".