I was 63 when I got my first cell phone. I seemed to manage quite well until then.
When they can pay for it themselves.
15, there's just no use for them before that
I got my son a cellphone at 8 (to replace the pager that he had had before) He promptly lost it. Fortunately a kindly soul found it and called my number.
I guess I should have waited until he was (say) 12.
At the age where they are needing rides to and from sports or other activities. Sometimes things can run short and need a ride early, or if the ride they were suppose to have doesn't show.
I think they should be at least 13.
My son was 10 when he got his first cell phone which was a flip phone. I'm a single parent and if he needed to reach me, I wanted him to have a way to do that. Not so many places now have pay phones and most of the time, the pay phones at the schools have been damaged. He played a lot of sports and I worked a lot of overtime, so we needed to be able to keep in touch on what was going on.
However, you need to base this decision on your own child, the circumstances why you feel they need one (not their own opinion) and the cost. No one needs an iPhone at 10 years old. That's a want, not a need. If they need a cell phone to keep in touch, a flip phone works just fine.
That is a determination for the parent to make. Age can be very deceiving. Some kids become responsible rather quickly, and others seem to never grow up.
My mom gave me my first phone at nine, and it was a Samsung. I don't think I even asked for it. I'm now a proud IPhone user, but I got my first phone young.