I'd say at about eighteen. I'm being facetious. I'd say from around twelve, with the proviso that they only carry a pre-paid phone which when the money runs out they can't use again until more cash has been paid in advance. That way they can't run up a big bill for Mum and Dad to pay. Also, they'd get an idea of just how expensive cell phone calls can be.
They would also learn to use it judiciously, and not waste in on frivolous, gossip-type calls.