I'm just going to keep it simple and go with the "decade rule" (don't care if there isn't one, I'm making one
). 10 years is long enough for most of a generation to move into the next phase of their life, and to look back and think "Ooh, I remember this when I was 'young' ".
2003 would make earlier PS2 and Xbox games retro, and that works for me.
Maybe it should be 15 or 20 years. In a previous job, I was told that retro (concerning goods in general) is 20 years or more and antique was 50 years. Not sure if it's gospel, but again, I'd just like to keep it simple. It's neat to think that my Nintendo and Sega games now qualify that category.
Retro style games are another topic entirely, since I see the target audience often as fans of the source material (modeled after games or systems that came before it). Retro in spirit but not in release. For example, I don't consider Pier Solar (Genesis... +Dreamcast/PC soon released) retro, as it's a newer game.
...just another lost soul...