What Age Is Too Young for a Cell Phone?

While I was shopping yesterday, I past a toddler talking on a cell phone up and I thought "I hope Mom had the sense to lock it before giving it to her little one to play with".  I glanced up to get Mom's attention in order to let her know that it appeared that her little one was actually talking to someone on the phone and noticed that she was talking on her cell phone.  Mom laughed at me and stated that that was her baby's cell phone.  A toddler with a cell phone of her own, really?  When is enough, enough?  I am not sure the exact age a child should have a cell of their own but I am pretty sure that it comes after their ability to identify numbers.  Children in most elementary schools are not allowed to bring phones to school, and will lose them if they are caught with them out of their backpack.  Some divorced parents would argue that they would never hear from their children if they didn't have their own phone.  So the debate is on, when is the appropriate age for a child to have a cell phone of their own?  One thing I think is important to consider is whether your child understands how billing for these phones works.  If they don't you may be in for some outragously expensive phone bills that you are ultimately responsible for.