My brother is also 12.
I have been very explicit with my parents that they do not know about the evils of technology as much as me, since I have been born in it.
It is extremely important for the children to not use it during puberty. It will scar and mold their brain for life.
My parents have not completely forbid him from using it as I sugested, because they are afraid to upset him and they truly can't imagine their own lifes without smartphones so they think they would be doing extreme harm that way. When the opposite is true.
What I have found to work is to ask my parents to enforce their authority for him to do things that I also do.
He's joined Muay Thai classes, walking the dog, spending the weekend with the familie, enforcing some curfew for the internet etc. These are things that I also do.
To sum up, my advice is to not expect you'll get what you want without being confrontational with your parents and demanding him to do activities you also do.