I think any age is fine as long as its on certain conditions. Like for some kids, They get one when they are like 7-8, but maybe because the parents were at work. Or for me, I had to get one when I was like 9, but because me and my family traveled a lot. Plus in this society, there so many dangerous people in the world, so I think 10 is a good age.
Well I have offered to pay for my own, offered to get a lame phone and let my dad pay for it, given many many reasons I need one and my dad still won't give me one and I'm 15. So I guess it's upto you!
I think the age depends on the need for a phone. If the kid is busy with extra-curricular activities & needs a way to contact parents, guardians, and other people involved then a phone is a good idea. Also, if a teen gets his/her driver's license & is away from family & other supervision often, a phone is a good form of contact. So, a good age range is 14-16.