The official canon age of Red and Blue are 11, it was in the manuals for the games.
I, personally, use any age set: A.D, for example, starts his journey at 11, just as anyone in the Japan-based areas can. Unova, and presumably any America-based area like Orre, start at 16 or later. Sometimes, like Michael shows, younger people choose or are forced to ignore the rule. The rule doesn't apply to battling, though, just journeying.
Basically, I'll happily use anyone over 11 or 16 =P