I generally think of anything 10 years or older as retro but that isn't a hard and fast rule. Shorter lived platforms become, or at least feel, retro much sooner.

I think the retro scene started earlier than 2000 though. It had definitely started by the early 1990s. I remember seeking out an Atari 2600 at a flea market around 1991 to replace the one I had gotten rid of ~5 years earlier when I got my Commodore 64. That was at least in part out of a sense of nostalgia. Also, if you look at when emulators started becoming a thing that was in the early to mid 1990s too. Activision's Action Pack (basically a 2600 emulator) was released in 1995, the first version of MAME was released in 1997. Nesticle, the first NES emulator appeared in early 1997 too. The platform I cared about the most at the time, the Commodore 64, had its first emulator released in 1993. I think the 8-bit computer retro wave hit before the video game retro wave did but I remember online auctions via usenet newsgroups in the early and mid 1990s among collectors for this stuff. Retro was definitely a thing then even if it wasn't nearly as big as it would ultimately become. The internet becoming popular is definitely a big part of what drove up prices though as it became easier to match demand with supply.