I'd go even further than this. No handheld device until 8 or 9 (if that, why is it good for them?), brick phone for secondary if totally necessary (long/unreliable commute), smartphone at 14 at the very youngest.
There's so much research that screen time is bad for us as adults, and absolutely terrible for children. Talk to any teacher and see how concentration has completely and utterly evaporated as a reliable presence, for every single lesson and every teacher. Emotional regulation, confidence and resilience, communication skills, self-entertainment and creativity, all of them have decreased massively and measurably since tablets and smartphones became the norm for developing young plastic brains. The more we drill into them that they are in TERRIBLE DANGER by talking to people and going to places IRL, and we need to watch over them and GPS-tag them at every possible moment, the more they'll be afraid and feel they need someone to watch them, supervise them, and tell them what to do. How can they develop independence and confidence when every action from their parents gives the opposite lesson?
Ours don't have smartphones and have busy, active, outdoor social lives and hobbies, organised face to face (younger ones) or over email on the house computer (the older ones, where time is limited and they don't just doomscroll or lose hours to SM sites).
In twenty years time, I'd be amazed if we're not scandalised at how common it was for adults to give children these devices. The greatest thing you can do as a parent now is to be the responsible adult and say no.