My sister owned a cellular phone store and then later my very close cousin where I had been employed for the last 3 years up until last Nov. but I have a slightly diffrent opinion then other on this subject.
My daughter is 9 and has what she "thinks" to be her own cell phone. Only pre-paid minutes which only have to be added every 3 months and what wasn't used before rolls over with new mins. for another 3 months (so not very expensive). I do make her buy her own $10 cards with what money she earns from family members or birthday money ect. I control the phone and she don't carry it with her unless I ok it but I'm telling ya it has really been a piece of mind for me.
For example I was getting out of the shower one day and she asked if she could go to the neighbors instead of rushing out of the bathroom i told her to take the phone and call me once she was inside the door she called 3 minutes later and I knew she was safely there.
Right now I don't let her go to many friends houses but I know I'll have to let go some as she gets older in which case she will have her cell phone and be able to sneak off in the bathroom or whatever if need be weather she feel uncomfy and wants to come home with out making friend feel bad or if she just wants to let me know all is well.