Kids might need parents help. They might be lost in a mall or some strange person came by and asked them to get in the car. You kid could call the police if that happens. Or if he/she is lost in the mall they could call their parents for help. Maybe the forgot their shorts and they need to use their phone to call you. That's not just it. What if they need help on math and you had to go to a meeting? You kid could call a friend for help. Maybe their best friend moved away. They might want to stay social with them bye messaging them.
Young kids today might not know yet about the dangers of eager hackers who would want to get the info on your phone. Kids who are unaware of such fall easily to attacks and have their info easily carried away, which can mean big trouble for things such as their bank accounts which are vital later in life. Imposing a minimum age would help avoid the risks of hackers getting into phones and stealing information you wouldn't want them to have.
Cellphones are known to have been tracked, some cases state that someone spies through your front camera using various apps. The game "Talking Angela" supports this, the game was believed to be spying on people through the face recognition feature in the game, I doubt any of you would want your children being spied on, there are also so many acts of suicide over cyberbullying and if young kids encounter cyber bullying without much knowledge, they could do something rash. Children should br prepared for the internet and cellphones, they should have enough time to prepare for the risks of owning an electronic device. 12 is a good age as that is when the pubescent stage of a child starts and they start going on their own.
That age in which a child has a cell phone ins controlled by their parents and the government definitely should not be the one controlling that. There are parental control settings for young kids, and parents can also not give them a smart phone that can have apps and all that stuff people are worrying about but just a phone for reaching others.
Parents are responsible for deciding when their kids should get a cell phone. A cell phone can be a good way for a kid to stay in touch with their parents.
Some "parent control" settings should be available (and probably already are) if the kid is on the parents' plan so then the parents would have to approve new apps so the "spying over the phone" issue wouldn't come up.
They can be set up to only dial their parent's number in case they need help, need to be picked up, or to inform their parents of their whereabouts. They can also be tracked by parents of smaller kids to make sure they are where they're supposed to be, and to find them if they are lost. They can also be used by kids with distant family member who want to keep regular contact with them. Cellphones are a tool and when used appropriately, can benefit both the kids and the parents.