How old is appropirate for kids to have their own cell phone?

Lost Art of Talking. Unless a child needs a cell phone for safety, i recommend waiting as long as you can. Nevermind radiation, cell phones can pose a hazard to personal safety; academic achievement; and, importantly, normal, healthy interactions with friends and family. Cell-phones also tend to distract from higher priorities, and, with web access, they open the door to potential moral decay.
Large variation. The answer to this depends on your family values and your family dynamics. Probably, there is no need at all for a child or teen to have a "smartphone" until they have a job where they can pay for the phone. On the other hand, it may be completely reasonable for a school aged child to have a cell phone to communicate with parents and family. Beyond that, there is no absolute answer.