For me it was when I joined a sports team and the coach left kids at the school alone. In my school district middle school and high school are in the same building and middle school is 7th&8th grade so probably like 7th grade
My 15 year old son can't stop washing it with his pants or losing it, and when he hasn't ruined or lost it, the battery is dead. But my 13 year old daughter always knows where hers is and it always has a charge.
My son, however, only ever uses it to let me know it's time to pick him up from band shows and rehearsals, and my daughter texts relentlessly.
I'd say if the child is careful with his or her things, shows restraint in use of electronic devices and has a need to be in communication with a parent, then there's good reason to buy them a no-frills phone.
We'll this is my opinion but i think you should wait until the child is responsible enough & proves to you that she is responsible to get a cellphone. weather the child is 12, 13, 14 or whatever that doesn't mean that they are responsible for a cell phone. Especially a 12 year old there still in that kid phase maybe when there 14. they'll be mature by then hopefully. I hope this helps if not then i don't know.