Yeah, definitely the 5th gen. It's been two decades plus since their release, so I think that's fairly enough time to consider them "retro". Although the 6th gen is nearing two decades, so by my own logic it could be considered retro, too.
I don't know; I look at the PS2 for instance, and can't seem to think or see "retro" in it. At least not yet. The PS1 on the other hand, at least to me, does have a "retro" feel to it. So yeah, mark me down for the 5th gen, too.