Thise days we see little children with phone, who habe access to unlimited internet, social media and all other things what they shouldn t have access. Its not an easy task to decide when they should be able to have their first phone.
I believe it have to be related with the moment when schedule can change at school, to warn their parent, or when they able to be alone without parent if any trouble to be able to reach them at any moment. But this depend of the parent, the character of the child, the area where they live.
There is many factor who will differ from one family to another. But as most of phone have internet now, we should limited internet for until a certain age, its crazy what they can see there and its very important to protect them from it, especially from social media.