It depends on each situation. My kids got their cell phones when they started high school. That would put them at 14 or so. They all played sports and would need to call to be picked up after practices that often ran long and road games we were unable to attend.
Their social life also expanded in high school and they would need to be picked up after games, dances, study groups, club activities, etc. Basically, once their schedules were more dependent on their activities than they were on Mom and Dad it became a necessity for us.