In the past, two generations back seemed to be considered the starting point for classic. For instance, when the 8th generation systems (Xbox 360, PS3, and Wii) came out in 2005-2006, the 6th generation systems (PS1, N64, and Saturn) finally crossed into "classic", meaning that standard gaming stores such as GameStop no longer typically carried them and that there had become a very wide gap in technical specs between the current systems and the classic systems.

8th -> 6th gen (Xbox 360, PS3, Wii to PS1, N64, Saturn)
7th -> 5th gen (PS2, GCN, Xbox to Genesis, SNES, TG16) Dreamcast is 6.5
6th -> 4th gen (PS1, N64, Saturn to NES, SMS)
5th -> 3rd gen (Genesis, SNES, TG16 to ColecoVision, Atari 5200, Vectrex)
4th -> 2nd gen (NES, SMS to Atari 2600, Intellivision)
3rd -> 1st gen (Atari 2600, Intellivision to PONG-like games)

There's a huge difference between each of these sets. With what we have now, 9th -> 7th gen (PS4, Xbox One to PS2, GCN and Xbox) there's still quite a difference. Though much less than the biggest gap (which would probably be 6th -> 4th gen) and less than most of the gaps listed above, there's still quite a large gap between the 7th generation and today's games. Today's games are often nearly photo-realistic and have much larger development budgets whereas the games of the 7th generation still look quite stylized and much lower resolution. Plus online play was in its infancy and the functionalities of the consoles were far more limited. For instance, my PS4 is used to get streaming television, whereas a PS2 is just a game console and DVD player. So there's definitely a big difference.

With that said, although the 7th generation shouldn't be lumped in with the current generation, it's clearly not in the same category as the 2D gaming systems. With that said, I'd be in favor of a three (or even four) category system for forums.

Classic gaming would be anything pre-6th generation - anything prior to the Sega Saturn, PlayStation, and N64.
Older gaming (or "3D era gaming") would be 6th and 7th generation.
Modern gaming would be 8th and 9th generation.

I could even see splitting "Classic Gaming" into pre-video game crash (1st-3rd generation) and post-video game crash (4th-5th generation).