In this day and age, with all the dangers a child faces i would say if you teach them to properly use it, i would get them one at 7 or 8 especially if they walk to or from school alone, i see kids around that age every morning walking to school. But i wouldnt buy a blackberry, i prefer androids.
Blackberries are good if you want to check email. Kids don't use email. They text EVERYTHING. A simple cell phone will work just fine. And I would only get a child a cell phone if they needed it to stay in communication with their parents. The rest us superfluous.
I know I may be old fashioned but I don't think any child should have a cell phone till they are 16, but I wouldn't get anyone a blackberry! They are way behind technologically speaking, I can't believe they're even still around!
A "child" should not have a Blackberry. In my opinion, Blackberries seem to be more for adults in business and stuff. I think at 12-13, a kid can get a phone, but not super oober fancy expensive. Tracfone, start 'em off with a Tracfone or something like that. :p