I think that since many if not most children in the US have phones at the age of twelve and it helps kids get along better if they have one as well. If they are mature enough to handle owning a phone without mistreating it or breaking any rules that you set for them. It is also useful for them to communicate with you where they are and where they will be.
A twelve year old is at an age where they will be placed in situations at school and activities where they are without an adult. If an emergency happens, it would be very beneficial for them to have a phone to call for help. It is important that your child has access to help in the event of an emergency.