I believe when they start to breed depends on other factors as well as age like water quality, ect.
There is no way to tell the difference between a male and a female. The best way to get a pair is to growout several different fish in a large (HUGE, if at all possible) tank, and wait until a pair forms. Even then, to be sure who is who, you'll actually have to catch them in the act.
Well, I suppose you could vent them after they're about 6 inches or so, but that's not even foolproof, as even experts make mistakes sometimes.