your captains are your leaders that lead by example. you pick the captains no matter what age/year they are and you tell the rest of the team what attributes they display that make them the captains.
IMO I would not normally let the team pick the captains because they may pick the most popular kids and not the best leaders. I pick captains by who works the hardest day in and day out and will lead by example...
Hmmm I've never been a fan of having Captains myself. I choose a leader to lead the team during warm-ups and thats it. IMHO it just creates too much drama.