My son is 13 and he has one.. mostly to keep in contact with family and such and play some games (he is not allowed to buy things for real money in them though)
there is an age limit on it i think its about 14ish. I know people who have made them for their kids that are younger its wrong children shouldnt be taught to share info with anyone
Facebook sucks at any age.
My thirteen year old is on it and we have to watch her like a hawk as she's not old enough to always make good choices .
Some stuff I see bothers me greatly.