As it is, baby brine shrimp can be a little big for new fries, so adult brine shrimps aren't a good idea. You'd need microworms, vinegar eels, walter worms, bananna worms, baby brine shrimp etc.
Age varies in petstores, but most are going to be somewhere between 3 months to about a year...
Bettas are naturally agressive. I do believe around 2 months is when they get aggressive enough to need to be jarred/removed from the grow out tank. However, it depends on the fish itself. Some of the boys might be extremely agressive where as others might be more relaxed and not need to be jarred (unless you want to) until a later day.
would be a good place to begin your research on breeding.