I think of "retro" as pixel-based and early 3D games that were sorta killed during the PS1/Saturn/N64 era.

Once we get into more advanced 3D / Polygonal games (PS2/GameCube/Xbox), it's a different era; but it's not due to the visual shift alone, but the design philosophy and craft/code itself. I would consider them "modern" and point to series like Zelda as an example.

Games like WindWaker feel more connected to Breath of the Wild than it does to Link to the Past or even Ocarina of Time. And I think the same goes for series like Mario, Metal Gear, and so on.

I'm not interested drawing a hard line at a year or system. Something is retro if it feels old.

Once upon a time PS2 was new but now it feels old (and looks garbage compared to new games) so it's retro.

~15 years is the arbitrary cutoff that feels right to me, despite how old it is continuing to make me feel

Personally I classify everything older than 30 years (in video gaming) to be retro-gaming i my mind! So even the first Pokémon games are retro to me :)

The answer is always "whatever generation came before I started playing."

@InkstainTheBat most people draw the line at about ~3 console generations ago. So now anything in PS2 era and under is considered retro. As people get older, and console generations are longer (see: Switch, PS4) it tends to get a bit messy.

I'm not really sure I consider anything retro? Old tech ends up still getting supported, just by indie hobbyists. But outside of that I guess XB360 and below since it's not being produced anymore.