Now this is a tricky one to define. Every gamer has his or her own definition of what makes retro.
I've seen gamers who claim that only the 8-bit systems are 'retro' (NES,SMS,C64,ZX Speccy, etc), I've seen those who have retro cover the 16-bit systems too (SMD/Genesis,SNES,PCE, Amiga, etc). And some even allow the 32-bits in as well (SS,PS,N64...I know, its 64bit but it comes from the same generation and its my blog so nerrr!).
Me? As far as I'm concerned anything up to and including the Dreamcast is retro. Why? Simple, really: If the system is commercially dead (ie you can not walk into a retail outlet such as Game, gamestation, HMV) and purchase a new game, then it is a legacy system and as such 'retro'. I'm sure many folks will disagree with this, which is fine, but that's my definition.
And, for what its worth, I'm ambiguous about classifying the original Xbox as retro just yet. Even though it is 6 years since it was effectively killed off by MS, fir some reason it just does not quite feel 'retro' just yet. The same goes for the GameCube.
Naturally, this definition is subject to permanent fluctuation, as both the Xbox and GC will undoubtedly enter the 'retro' classification for this blog sooner or later, Probably when the WiiU arrives for the latter.