No one needs a cell phone.
At any age.
Cell phones are a luxury. But they are a luxury, one could argue, that children "need" more than adults do. If an adult gets stuck in a bad situation, an adult can drive himself or herself home. An adult can usually figure things out on his/her own.
I'm guessing you don't have a child in middle school. They do more than just go to school, get on a bus and ride home. They have school dances, after-school activities, sports, piano lessons, projects at the library...and broken bones. They have bullies, sleepovers, divorced parents and any number of valid reasons to call home.
My 11 year-old daughter has a cellphone -- a barebones, $25 SonyEricsson...and I thank goodness she does. She has been extremely responsible with it since she got it 6 months ago. It still looks brand new. She doesn't abuse the privilege of owning it. She uses it to call me, her dad and her grandparents. Occasionally she calls a friend for homework help.
Heck, I've broken a cellphone in fewer than 6 months myself! I cracked the screen on my iPhone 2 days after I replaced the first one. I should let her carry my phone.
Don't underestimate children...their needs or their abilities!