My gymmie just turned 10 and got one for her bday (my dhs "old" phone, a then top of the line EVO phone.... Now probably hopelessly outdated since it's like 3 whole years old..!). My older DD got her first phone right before her 11th bday. My reasoning is that they are/were at places for several hours without me, and I would like for them to be able to get a hold of me (and vice versa) if necessary. Sure, at the gym or studio they could go to the office and borrow the landline to call me, but it wouldn't be able to get to them without making a hassle for others. We also do not have a landline at all, so their cell phones are the only way for me to connect to them if I leave them at home to run errands etc. they both have unlimited calls and texting, as well as data access... bUT they know that I can, and will, check messages and everything else they do on their phones at any given time and if I ever see anything I don't approve of, the phone is gone. If someone misbehaves, the phone (the fun part of it) is gone.
They both have kindles/iPads as well as iPod touchs... They do have to treat their electronics well, and I'm a stickler for having things charged and accessible. The whole point is to make the girls and I more connected and easy to reach, and if they use up their batteries playing games and messing around and I can't reach them, that's no good.
I never thought I would allow my kids to have cell phones at such a young age, but the reality of it is that for us, it makes things easier and it's really part of the times.