Some people consider retro gaming to be whatever they were playing as a kid, and whatever the new stuff is to be modern. Because the PS360 gen lasted so long, that means there are a lot of younger people who will consider it retro because 10 years ago is literally half the time they have lived and feels like antiquity. If you are 20 years old, and first played a system when you were 10, that's gonna seem "retro".
However I think those systems are simply too modern and similar to their successors to be really retro. For example I would not call a gtx 1060 retro just because we have 3060 now. But a Riva card with a pentium II? Now THAT's retro! So many PS3 games were ported to PS4 just for a performance bump. Most PS4 games would be possible on PS3 with a performance downgrade. That only falls into retro if your definition is "anything before current contemporary games". Try downgrading PS2 to PS1, yeah right.
The farther back you go, the bigger leaps were made with available technology. As you advance, progress became very slow and iterative. In my opinion, Dreamcast was the last big jump and everything since has been a slow progression of essentially the same thing. It marked a point where 3D graphics and game design left the janky antiquity of N64/PS1/Saturn, which I consider retro games.
I have always thought of the N64 as having the same role for 3D games that NES had for 2D. Mario 64 was a revolution that defined 3D game design the same way SMB did for 2D games on NES. Both systems, while a big deal for their time, were totally and utterly dated as soon as their successors came out. 64 is as much a retro 3D system as the NES is a 2D one.
The 64's successor, Gamecube, ironed out all the primitive first-gen-3D jankiness of the 64 and even had launch games like Rogue Squadron II that still look fine today. Much of its library could just get a resolution and framerate bump and be perfectly playable today for a modern audience used to current games. This applies equally to PS2 and Xbox.
So I personally consider 64/PS1/SAT and anything prior to be "retro". Anything newer is either modern or just playing on a dated system. PCs from 2010 are not retro, they're just dated.