If both you and yours Parental Units (Parents, Grandparents, Close Family Friends, ect.) Think you are mature to use it or not. So there really is no age limit, for my cousins who are 8 and 9 both have facebooks of there own.
The COPA law requires 13 and up. This is to protect children from predators who will "friend" them and then lure them to a meeting and harm them.
They put 13 because they are considering the fact that quite a few 12 and under kids haven't been taught how to protect themselves by their parents. Of course, a lot of 13 and up teens are like that as well.
Some kids under 13 ARE responsible because of how they are raised but it's still a good idea to wait until they are 13.