True story...my son who just turned 4 asked me the other day if he could get a real cell phone (he has a fake one)for his next B-day, when he turns 5 (as he's holding up 5 fingers).
I say if the kid is responsible enough and you are able afford it, then yeah the kid should have a cell phone. What you want to keep in mind is that every situation is different. My niece and nephew who are very active in sports and other activites have had a cell phone since they were 9 years old. Here's the difference, it's no big deal to them or if anything a hassle to them to have to take it with them and they are responsible enough to take care of it and not abuse or use it when it's not needed.
The whole, "I used to walk 10 miles in snow, up hill everyday to get to school and I didn't need no damn cell phone" is like blah blah blah to me.
To each his own.