I would consider anything pre-PS3/Xbox360 as retro. Anything after is old but I'd still consider them modern games. Aside from graphics and scale not much has changed since the PS3 era.
I'm relatively with you there. PS5, Xbox Series, and Switch are the current modern consoles. When I was 12, we got a gamecube while it was current gen, and considered N64 to be somewhat retro already, but SNES was firmly retro- or 2 Gens back. I think it's reasonable to not think PS3 and 360 are retro, but older than that surely is. PS3 and 360 games don't lag behind modern games by the same leaps and bounds SNES to GameCube did. But PS2, Xbox, and GameCube are all still in the pre-HD Era. For that reason I'd make the rather radical suggestion that Wii might be considered retro already, since it remained an SD console while it's contemporaries were HD.