We gave our boys cell phones when they started participating in activities (music lessons and longer sports practices) at which we would not be present (age 10, I think).
It has been very helpful to know that if for any reason we are running late in picking them up, we can send a text with that information. They also feel more comfortable having a quick way to reach us when they are out without us.
Both of them started out with a pre-paid phone, and they worked beautifully for this purpose.
My older son is now 17, and he pays for his own more extensive service.