I don't know. We purchased a family plan and gave my teenager a cell phone when he started driving (so he was 16.) He's 18 now, and I think all his friends have a cell phone.
On a positive note, I think a cell phone can be used to teach a certain level of restraint and responsibility. We have a family plan that allows free minutes between family members (the reason we purchased the phone was for "us" to be able to communicate.) We intentionally purchased a low amount of minutes, 400 per month, and all of us share those minutes, so the "teenager" knows he has to use restraint in making calls.
In the 2 years we've had the plan, we've only gone over our "minutes" once (and that was due to lots of calls being made during a medical emergency within the family,) so I think there is a positive side that can be taught through budgeting cell phone minutes, only talking when absolutely necessary, learning to exercise restraint, etc.
I have 4 teenage daughters and they all have a cell phone too.
It has allowed me twice in 5 years to figure out that they were not in the place they were suposed to be. If they don't answer they are in trouble.
We live in Richmond where there is always a signal so they can't use that excuse.
I did the same. My company pays for my phone and all I need to cover is the extra line fee for my wife and 16 YO daughter. My plan has far more minutes that I ever use, but I'm not telling her that. She's very mature in some areas, so I don't anticipate any problems. Also, our package does not include text messaging. If she wants that, she's paying for it.