High school aged children definitely can understand the concept. I would argue middle school aged children can as well.

High school aged children are well known to be complete and utter dumbasses, especially when it comes to making mistakes who's consequences are abstract and long-term in nature. Punishment by social superiors is oftentimes the only thing preventing them from doing idiotic things, because their brains are not developed enough to think very far into the future. And even then, proper impulse control is one of the last things a developing brain develops, so they might understand the issues but be psychologically incapable of the self-control needed for it. Not to mention, social media apps are designed by psychology experts in Silicon Valley to be as addictive and distracting as possible, since that's how you get people to use your app. Having those in your pocket, when you're too young and dumb to understand the consequences of overusing it, and can't even exercise self control when they're pointed out to you? It would be irresponsible for us adults to continue allowing it.

Again, if the parents are worried the kids are spending too long on their phones they can do something about it, not the gov.

Parents aren't worried about this, and that's the root of the problem. If the school system does nothing about it, then the kids will just end up addicted to TikTok and completely unprepared for the world on account of being distracted in class. Their parents aren't going to do anything about it until it's too late.