When you are old enough to know what to talk about with out sticking your foot in your mouth, and when you can pay for your own cell phone bill and when you know when it's time to shut your mouth. That age is based on how fast one matures and is able to take on that kind of responsibility.
I got mine at eight but I would say 12+ because usually younger kids always bother people constantly calling and texting people non-stop and not even knowing the value of a phone. I think you're old enough to have a phone.
If you have an after-school activity, club, or sport that requires a parent to pick you up, if you live in a hard neighborhood, or if you are 12+ and responsible, you need a cell phone. My question is how to convince unwilling parents to get you one if you fit into one of the above categories.
A person can start having a cell phone once he's mature and responsible enough to buy one and pay for his phone bill. It's also wise if parents let their teens have it because teens are most likely not at home often, and cellphones are handy to keep parents from worrying.