We bought prepaids for our nieces much younger than that. I think they were ages 5.5 and 9.5 that Christmas. The thought was that they could learn to use them by May for our vacation to Disney. If they were lost the preprogrammed numbers could be called to reach those on the trip with them (aunts/uncles/grandparents/parents).
The 9.5 y/o took to it like a duck to water. By age 11 she got a new phone on her parent's plan. They now have no home phone and the 8y/o and 12y/o are on the plan. Cell phones are a privalege in the household, not a right and they cannot take them to school and the phone doesn't go in their room at night (cuts back on late night texting).
My 15y/o niece got her cell phone at around age 11 or 12. She now texts into the late night. I often text her around 10pm and joke that she should go to bed and get some sleep.
I have a feeling mine will have some kind of phones somewhere between 6-10. I'd prefer they have an know how to use one by the age they go on play dates alone and sleep overs. Sometimes asking another person's mom or dad to use the phone to call home is rough for kids.
Funny how my thoughts on this have changed. i remember freaking out that my 12y/o niece got a phone for her room when she was 5y/o. her mom gave her a list of numbers and taught her the names. It was so funny to get calls from her at 5...several times after her bedtime...whispering...just wanting to talk. Anyway...they are social beings....as long as they can be reasonable minutes-wise...any phone can be a learning tool.
I do agree with LEMS...with younger kids it is easier to be sure they have it when WE the parents feel they should...like the 8y/o niece. She has it when not with her parents, so she can call home and there's no texting...not that she would.