Retro/old school and classic to me are totally different in my opinion.

I'd consider retro games at most 8bit or prior. Classic would be anything, as of right now, 64bit or prior.

To me, retro isn't ever going to define anything beyond 8bit. It's where I started my gaming experience so it, to me, was when gaming first started. If I was older and started gaming earlier with my main console was the 2600 or something else, then I'd probably say that it would be retro and anything beyond wouldn't be.

Classic doesn't define a beginning for me though. It's anything that could be considered archaic to todays standards.