This is a sticky question. For most it is merely a matter of personal choice, so we will forego right or wrong answers. The better question would be, "Does it make sense for my child to have a cell phone?"
Let's say you have a ten-year old child who is in many school activities and needs a way to contact parents, friends, or parents of friends, for important reasons: This would make sense. If you have a ten-year old child who is not in any activities, spends a majority of his/her time at home and hooked up to electronics, then no, it probably would not make sense. The choice is yours, however the latter scenario may deserve serious parental introspection.
Kids want cell phones for various reasons, and some of those reasons are perfectly...well...reasonable. But some children want a cell phone because their friend, their friend's friend, their friend's friend's younger sister, and their friend's friend's younger sister's best-friend (you see where this is going) all have cell phones --or simply because cell phones are 'cool'. In all honesty, cell phone are pretty cool! If they were not, upgrade options with cell phone providers would not be so popular among parents...those same parents who are so quick to post a photo of their newest and brightest handheld gadgets on Facebook status updates. I'm sure more than one parent has thought to themselves, "How can I take a picture of my new phone with my new phone?" (Answer: The infamous bathroom pic in front of the mirror. Which will likely be superseded eventually with a pre-programmed picture readily installed for public sharing --if they haven't done that already. Or just maybe, cell phone companies will post it 'on your behalf' because they know how much we like to share important information with the world.)
Some parents think a child who is seven is old enough. Some parents think a child who is twelve is not old enough. Some parents will tell their children they are old enough to have a phone when they can pay for it themselves. In all regards, it is not okay to question other, seemingly responsible, parents for their choice in this personal matter.
So, once again, the question, "At what age should my child have a cell phone?" It is the author's opinion that a child is old enough to have a cell phone when the parents believe it serves an important and functional purpose in the child's life.