Classic gaming has been around only since about the 1980s, as far as I'm concerned.
This is for consoles, but handhelds can be included as well.
I'd say the PONG consoles became classic around the video game crash (1984).
2nd gen? For Channel F and Studio II, mid 1980s. For the Atari 2600 on (the "true" 2nd gen), I'd say it was circa 1990 when they became classic, with the end of production of the 2600 Jr and the Colecovision, Intellivision, Vectrex, etc having been out of production for several years.
3rd gen - I'd say the Master System and other non-NES systems, due to their earlier decline, entered "classic" around 1995, when the Master System had been twice replaced (Genesis, then Saturn). For NES, I'd say it was at the end of the 1990s.
4th gen - I would say around 2002 for Genesis/SNES, near the beginning of the 6th generation cycle. Non-Genesis/SNES systems (including Sega addons), late 1990s.
5th gen (pre Saturn) - These were the "fake" 5th gen systems (or "4.5 gen"). 3DO was the most successful of these. I would say about 2000.
5th gen (true) - Saturn, PS1, N64. Saturn went classic first, around 2002. N64 was just entering classic when I started collecting, so 2005. PS1 was around 2006, when PS3 launched.
6th gen - Xbox and Gamecube, 2010-ish. PS2 is just now classic, so 2014 or so.
I think of each console as having a lifecycle.
New - Young - Peak - Mature - Old - Classic.
Let's use the PS1 as an example:
New - Sept 1995 to 1996. Console launch, first couple batches of games. Limited adoption.
Young - late 1996 to end of 1997. More games, more players, more gotten out of the hardware. First price drops.
Peak - 1998 and 1999. Large game library, high market penetration, systems value priced.
Mature - 2000 and 2001. Large game library, high market penetration, systems cheap. Console being superceded - in this case by Dreamcast, PS2, Xbox, GCN.
Old - 2002 to 2006 - System superceded, but still sold in background. Game releases slow dramatically and eventually stop. Production ends.
Classic - since 2007 - Places like GameStop no longer sell the system or games as a general rule. Pricing and market moves into the classic realm.