Depending on there you live it could cause additional risk for your kid getting mugged because of it.
I know most 7th graders have a cell phone but, but since it's a very expensive and very desired phone(for thieves) and if they see him/her playing around with it in public, it's very easy to snatch it off a child. Or even someone from school could snatch it out of the schoolbag during P.E. etc.
Honestly it would make me worried.
About usage there should be no problem, this generation grew up with Computers, PSP, etc and can be often smarter with technology than most adults.