Funny you asked this question as it haunts me right now. My fifth grader is constantly nagging me for one and contends "everyone else has one." Oh brother...
There is no "right" age to allow your kids to have one. In my opinion, it depends upon the child and the activities the child is involved in. I can't imagine a need until my kids are much older, well into middle school or beginning high school. Many parents claim they need to know where their kids are. I contend fifth grade is WAY too young to not know where they are.
One of my daughter's best friends is a gymnastics superstar who travels for meets. That could be a unique situation, however. Her parents want her to check in with them when she's traveling.
I'd get her a very simple one that can only call one or two numbers, can't text, has no internet access, etc.. Anyway, since she's not my kid, I can only say I will wait for as long as possible as I've heard of "problems" with cell phone use. The internet's bad enough.