I believe anyone under the age of 11, Possibly 12 don't need a phone. Most of the time kids that age just use them to play games, So they should just get an iPad, PC, Or console instead. Children that age don't need to have easy access to the internet through messaging and browsing because they aren't ready to deal with the people and the things they will encounter. Anyone 13 and over though should have one though, Because by that point they may want to do stuff like go out with their friends without parental supervision, So it would be good for them to have a device to be in contact with their parents. They will also hopefully by this point be responsible online and not watch mindless toy review content on YouTube.
Let me explain. The phone must be limited so that it can only make calls / receive calls. This is actually possible using smartphones (app lock), But for simplicity, You can give your kid a stupidphone. No games, No videos, No internet. Just calls, In case they need something or there is a problem.
After a certain age, There can be less limitations on the phone, Since the kid understands more and is more responsible (depends on the kid though :D).
One other idea instead of locking the unnecessary apps it could be a surveillance app, So that the parent can see what the kid does all the time / how much time is spent on the phone etc. Then it's up to the parent to decide what to do.
Anyone age 9 and up would likely be responsible enough to have a phone 📱. They can call their parents when they want to be picked up from school. They can also contact their friends if they are feeling lonely. Sometimes the kid needs to wait for something so they can play on the phone until a parent or friend comes. They should have a phone if they’re responsible enough. The parent can put restrictions on what apps they can download and use
Tho the word "child" is vague as it could mean anyone from the unborn (with child) to fully grown adults (my children have kids of their own), I will use the legal definition of human between the ages of birth and puberty. This is basically grade school and younger so they would have responsible adults/teens around at all times. If some situation comes where a call needs to be made, This older person could and would make such a call so the young child does not need their own phone of any kind.
Kids, Especially the very young, Get distracted easily. Getting kids to eat or do chores is hard enough without them having some distraction within easy reach.
Creating social skills is important to early development and by phone is no substitute for personal interaction. Today, We see teens texting each other within the same room when they could be having an actual conversation. Is this trend something we want to encourage?
Children are spending FAR too much time nowadays on mobile phones. Sure, Technology is moving on, But then children are growing up far too quickly as well. I don't see why if kids want to meet up, They can't use the telephone. I mean, That's what has been used before and it worked fine, So why use this technology?
Social media should have proper age limits too. Children are being told what to do by a screen. It's simply not healthy. Phones should have age limits when buying.