For me its about 10 years after release. "Retro" is just old and cool and a lot of games are now old and cool.

So for me the PS3/360/Wii is transitioning into retro.

I'd consider them just old.

Anything last gen or older. Retro doesn't mean ancient. I have everything from the Atari 2600 to the Wii U in my retro collection.

I'll kinda of arbitrarily say it's 1 year after the last official game is released for the platform.

I know this isn’t popular nor what most people would agree on, but to me retro is pretty much synonym with 8 bits (maybe some early 16bit) and the arcades that were around at the time. May have to do with me being old, but I think it is more because I felt a shift in look & feel, from sprites, pixel art, side scrolling, moving room to room, etc. to a more “modern” feel, with 3d models, etc. of course now we have a bajillion more polygons, shaders, ray tracing, but it is just a change in degree of the same thing.

Of course you still get games nowadays that look retro, but it is done out of artistic choice, not necessity. The constraints of the old platforms led to truly inventive solutions.