Chundles said:
5 to 8 years = retro
8+ years = old.
I also think this is backwards, and missing a category.
OLD: A Blue & White G3 is old, not retro, because it can still perform modern tasks and run the latest OS. It just does it rather slowly now. 1998-2002
RETRO: Retro is when a computer cannot run the current OS or any current web browser, but it can be useful if you really want to (using older browsers, older versions of photoshop, etc)... A Retro computer is probably anything from 1992 - 1998, such as a Power Mac 6500.
VINTAGE: A computer so old, it almost completely lacks any ability to perform any useful task in the modern world, and attempting to do so would be foolish for the amount of inexpensive Retro and Old computers is so plentiful, no level of cheapness (not even free) could justify actually using the device today. 1992 and earlier... such as the original Macintosh.
Of course, all of the categories have exceptions (Mac SE/30 may be placed into "retro" category) but I think this is a generally good set of rules. I still continue to use Mac OS 7 on a near daily basis for a variety of tasks.