Depends on the child, and the parents, where they live, and the like. But as a general rule of thumb is progressively adding more responsibility, freedom and access.
Start with a dumb-phone (as soon as the child is responsible enough to use it correctly) limited to emergency services, family members and trusted adults.
Let the child add friends, and schoolmates as the child gains understanding and responsibility. Limited internet access would be reserved for home and school pc.
Once the child has learned to handle the responsibility of chores, and has learned to handle the basic hazards of friendship, allow some self-autonomy with friend contacts, limited internet access on the device (walled garden for music and web). But allow social media contacts only on home pc to learn how to engage responsibly.
Once the child has learned to handle the responsibilities of summer employment, driving responsibly (driving age here starts at 14), and has started to engage in civic behaviour (scouts, 4-h, ffa, camp counsellor etc) and if the child wants to, allow them to buy their own smart cell phone and pay for with internet service let them, provisionally.
If my child is capable and not ready to leave home (or at least well prepared) to become a productive member of society between 16-18 -- including a smart cell phone, but all they need is the schooling, I have failed myself in my duties as a parent.
