If a kid is mature enough, does not treat it like a toy, and does not treat it as a social event, and does not go around giving out their phone number to whomever... and understands it is just for calling mommy/daddy/for emergencies etc., then per a reasonable age, they can use a cell-phone.
Parents give their kids cell phones (at least my friends do), for EMERGENCY use and because their kids are perhaps in after school programs and are not picked up until they get off work, often after 5:00pm.
AND, they give their kids, cell phones which has NO TEXTING on it. AND, the kids are given strict instructions... that their phone-number is NOT to be given, out to whomever. Because, Mommy & Daddy pay for it. It is not for socializing.
IF a child can understand those parameters, then it is NOT an 'entertainment' toy. It is a cell-phone. NOT a toy. NOT a 'thing' to have just because all the other kids have one.
It is a way to communicate with Mommy & Daddy. And for emergencies.
ONLY while at school. If not at school & they are with Mommy & Daddy.... their kids are NOT allowed to just have it 24 hours a day, nor go to bed with it. They are not allowed to have it all the time. It is still the Mom & Dad's phone. But it is for their child to 'use'... when needed and per circumstance. So, thus, their kids don't have the cell-phone with them all the time. When not needed/not in use, the Parent.... hangs onto it.
Then it is given to their child, when it is needed.
These are the RULES, my friends have with their kids, per cell phones. Ya know?
Common sense.
My Daughter is 8. She does not need one. Nor does she have one. But she is mature. I would trust her. But she has no 'need' for one. I am home with her, I am a SAHM, she is not anywhere just by herself etc. nor does she walk home by herself etc., and she would not need one. And I am with her at her lessons too.
To me, a 6,7 year old, is not responsible enough. They would at the least, loose it somewhere. For example.
For older kids, Latchkey kids, kids home alone or on their own or what not, a cell phone is practical. Not a 'toy'. But again, a child has to be able.... to use it, as per the RULES of the parent. If not, it gets taken away.
A cell phone, per young kids, is NOT an 'entertainment' center. It is not, for that reason. Taking and sending pics to each other on phones... yah, kids even send naked pic of themselves/others that way too.
Some kids, just cannot... handle the responsibility of a phone. Nor the use of it. NO matter what age.
One Grandparent for example, told me of a bunch of Teens eating at a fast food restaurant. They were all giggling. This was a bunch of boys and girls. One of the girls, was, with her CELL phone, taking photos of herself.... under her skirt... and sending it to her guy friends, at the table.
The Grandparent that told me this, said it was disgusting.
But well... that is one of the things, that kids can do with cell phones... which has photo and texting capability on it.
So, no wonder there are all kinds of problems... with kids sending each other lewd photos of each other and Bullying by texting on phones.
It is not a toy.
It is a phone.
Still.
There is a difference between a BASIC phone and a phone that has all the bells and whistles on it.
A kid, does not need all that.