How Old is Old Enough For A Cellphone

This question baffles me. I honestly can say I don't know. My Jacquie got her first cell phone at the ripe old age of 16. I thought this was an appropriate age. I still think that. My Nichole shared a cell phone with me about the age of 13. See, like I have said before, in my house Jacquie was considered T and E (trial and error) child. I didn't exactly err, but I liked to know that I could get in touch with Nichole and see if she was alright...
And I'll tell you what...when I told her to call in every 15 minutes or half and hour, she did it like clock work. I was always impressed by that. But see now that kept me trusting her to go off the block or over a friends house, because I could count on her calling in on those specific times, she never missed a call.
Now a neighbor of mine was trying to get in touch with her 5 year old son through her mothers cell phone or her grandmothers cell phone while they were on vacation and she couldn't always get in touch with him and this troubled her. So she got him that childs cell phone. I think it s called firefly or cricket or something. And this worked for her.
You tell me what you think. How old was your children when they first got a cell phone? Do you think you decided on the right age or do you regret it?