My son got his own phone when he was 8 years old. He stayed with hand-me-down phones until he was 13, when he got a phone that more fit his needs (texting). For me, I like the idea of my son having a GPS phone hidden on his person at all times, just in case of something really bad happened.
I also like the idea that my young child could contact me at any time. I got a call from the school playground one day, my son had something happened that triggered an allergic reaction. When that happened he was to go to the school nurse. The teacher on the playground wouldn't let my son go for some reason, dispute the note that was on file with the school that said that was the procedure. I know other children have died in similar situations. This turned out to be nothing, but it was good we were able to clear it up quickly, just in case.
It comes down to this, I want to have the extra security of constant, immediate communication with my son. Now that he is an older teen, I want the same exact thing, though for totally different reasons. Some might call young kids with phone spoiled, I called mine safe.