For that matter, how many kids manage their parents' devices. Maybe less so today, but for a long time, a lot of children were far more tech savvy than their parents. The contrast between my grandmothers when they were still around was stark. One never fell for anything... the other, I was cleaning malware it felt like quarterly.
My parents for a long time used their neighbor's wifi, despite having their own, because they didn't remember the password.
That said, having the carrier assign certain devices marked as "child" or "adult" or even with a DoB stamp that would change the flag when they became an adult might not be a bad thing. While intrusive would still be better than the forced ID path that some states and countries are striving towards.
That may have been a half decent excuse for parents 2 decades ago, but it isn't very good now when current parents grew up in the computer age with computers at school and in the vast majority of homes and even 90 year olds are using smart phones daily.
Like I said, likely less so today. That said, there are still a LOT of people that can use their devices, but have no understanding of security, configuration, etc. My SO can't even handle a password manager, but she can manage live TikTok chats/streams (or whatever they're called) with ease.
That said, it could definitely be done relatively easily at the carrier level, and a really simple addition to browsers, even if only on mobile devices.