We got our son one for his 7th birthday.
Our reasoning is similar, we wanted him to be able to call us (or for help) in case of emergency, and also to be teaching continuing levels of responsibility, and to foster closeness between him and family/friends that aren't close geographically (actually, we use skype for international calling), and we wanted him to have a gps tracker in case of REAL emergency (like getting lost, or abducted).
When I was a child, all of my neighbors had phones in their houses. As did we. (These days, not only do WE not have a house line, most of our friends and neighbors don't, either). Our schools had phones in the front lobby, there were pay phones everywhere, and EVERYONE let children use them. So if something happened on the playground at school (elementary) I could go in the office and call my mom and talk about it. It was encouraged. If my friends and I were playing and someone got hurt we could run to any house and ask to use their phone. If there was a fire or real emergency we were always taught to run next door and use their phone. If it was a BIG emergency, it was accepted practice that it was okay to GO INTO ANYONE'S house to use their phone, whether or not they were there. This was all in elementary school, K-4th in particular.
As I got older, I was allowed a lot more freedom...the "be home by dinner" kind of freedom. Mostly nothing happened... but I spent hours and hours exploring the canyons. There was the time I discovered 6 rotting and mutilated corpses (1 child, 2 teenagers, 1 adult, & 2 that I couldn't identify by anything more than the fact that there were skulls. All (well, 4 at least, the other two were pretty far gone) Caucasian. I ran. I was about 3 hours from home in a part of the preserve I'd never been in before. It took me 5 hours to get home, because I got lost. Sitting on my curb later, I decided not to tell anyone because I would never be able to find my way back & I didn't want to be disallowed from roaming the canyons. (I was 11, give me a break, I've regretted and agonized over that decision for years. But the point is, my parents never knew. My dad was military special ops, and had a brand new satellite phone, early 80's it was the size of "gee I hope they let me get this on the plane" carry-on luggage and weighed at least 50 lbs. I remember wishing as I stared at the bodies that they made smaller ones so I could call for help. Then my adrenalin kicked in, and I ran. And ran. And ran.). There was also the time that (in familiar territory) 2 years later that I was chased by a gang. 15 minutes had me in the safety of my friend Ricardo's (illegal) encampment. His dad & uncles & older brothers chased the gangbangers off... and I stayed there for a few hours while they fed me up, made me feel better, and then made sure no one was hanging around, before they escorted me home. Ricardo's family was a godsend that day. Although I often spent time with them, they saved my life (or my chastity).
Gosh there are about a thousand examples of times a cell phone would have come in handy in my own childhood (although the previous 2 were SOME of the scariest). But the time my girlfriend ditched me at the movie theater downtown, a fieldtrip where 2 of us were left behind by the busses at San Juan Capistrano, and at least half of the times a cellphone would have come in handy from age 6-14) there was a payphone we could use...with a live operator that would call our parents, and make sure they answered. I've forgotten my cellphone recently and searched in vain for a payphone. They've mostly been taken out to discourage drug traffic. I've also had a rather desperate 2nd grader borrow my (a stranger's) cellphone, because no one had picked her up from the bus stop, and she'd been waiting for over an hour.
Anyhow... our son has been using our cellphones since he was tiny. We nearly got him one when he was 5... but didn't because we knew we were changing area codes soon, and would need new numbers. When he was six we got the phone, but saved it for his birthday.
If he's at home with us he has to ask to use it. Out and about, he's only allowed to use it for emergencies or to coordinate a playdate, or to take pictures. He tested boundaries with it the first 2 weeks, but since then it's been golden.
Anyhow... my thoughts and experiences. :)
R