She is 15. She is a minor. A minor cannot sign a contract to purchase a cell phone. If somebody sells her one sue them. Actually it should be illegal to sign-up for a social media account before you’re 18, though as a practical matter I don’t know how you’d enforce that.
It depends on how determined they are and how much respect they still have for her mother.... they do find a way to do what they want.. 15 or 18..
they “borrow”.. they do whatever is necessary .. computers and phone are addictive to these kids and not having one leaves them not knowing waht to do with their time.. and they figure out how to get their way...
These kids need a job... and not spend their time on their phone...