13 is the age I think is best,they are a Teenager.
Any younger, is totally not needed....As far as a child having one to contact the other parent, while staying over....that's Not a great idea.
I have gone through that time, and my son would be phoning me all the time,
if he had -had one.
That is a 'time' there should be no interfering, as it is the other parents time.
Children sometimes can make a storm in a tea-cup....and it add's to the angst, when parents have separated.
13 was the age I gave my son one, and he felt very grown up.His attitude got better too.