It's a very fine line indeed.
I personally wouldn't put anything post Playstation/Saturn in it.
I don't consider these games retro yet, although games can still be referred to as 'classic' I suppose.
I certainly wouldn't consider a Dreamcast game to be retro in any way. Half of the games look the same as the PS2 and that's still abooot!!!
Just my 2 chocolate bars...
(Twix and Curly Wurly)