I would say up to the 5th gen. Games from the PS2 and onwards look and play much more akin to modern vidya without the jewry, with most of the hardware limitations on gameplay gone out of the way. You could show them to kids or someone who doesn't even play videogames and they'd think they look pretty cool, and get into them somewhat easily. But PS1, SNES, Genesis, Game Boy Color and maybe even some Dreamcast games? If i showed them to most people more used to modern vidya, they'd probably think they aged horribly and that they look and play like barebones tech demos of latter 2000's vidya. I'd have a hard time selling them to anyone who didn't grow up with those consoles or who isn't very patient without a "it was good for it's time" mindset.
The only exception would probably be the Nintendo DS, at least compared with the PSP. I also dunno where to draw the line with PC games.