Okay, cellphones. They are annoying as hell. And to see kids walking around with them (kids defined by me as anyone not yet out of high school), frustrates me even more. But let's not forget when the real push for these "toys" began. Do you all remember 9-11? Need I say more? Okay, I will. Many people were trapped in those buildings, and phone lines were jammed as loved ones called loved ones for their final words to each other. Disaster strikes, and usually, unknowingly. To have a cell phone in your pocket gives you the same sort of relief for such an instance, God forbid it should happen to anyone - but it does happen.
Here's the question I would tack on to your initial question. How would you feel if your child did NOT have a means to contact you in such a situation? And is AGE the only consideration when giving a cellphone to a young one? I think you'd be hard pressed to find someone who thought it was just a matter of AGE.
Okay, enough philosophy, however timid. Here's my 2 cents on the issue, for whatever 2 cents is really worth these days. Ready? It's not the age of the person. It's the LEVEL OF RESPONSIBILITY that should be considered. If you feel your little one is responsible, than I would say equip them with the tool, and teach them how to use it responsibly. If you cannot say with a definitive level that your child is responsible, than perhaps you ought to stay with the beeper method. Regrettably, you're probably the only person who can judge on this criteria, but I think it's the best criteria to work from.
And remember, no 2 people are the same. What might be right for one 13 year old, may not be right for another one. The problem here though is PEER PRESSURE. Kids want what other kids have. And they'll bug the snot out of you to get it. Good Luck in your decisions with your kids.