All of my kids had cell phones long before I did. But I did not pay for them.
Now, that they are so prevalent, I would think, that once your kids start going to the ball field or the mall ( etc) with their friends rather than with a parent or other adult family member, would be a good time. If parents are providing the service, it requires parental control and monitoring.
For the younger ones, I would consider one of those plans where the child can call me, a grandparent, favoured aunt or other trusted person.
Kids do not need a phone on their hip in class or while mowing the family back yard! They are not responding to emergencies and don't need 500 hours of unlimited talk and text.
Does your child NEED a phone? Are they coming home to an empty house with no land line? Are they walking to the park or home from school? Do they have an after school job to pay for the service? Like curfews and driving lessons, the answer depends upon the child and the family unit.