You want to prohibit anyone under age 21 from posting to or using TikTok. That is regulation. But you aren’t just talking about one platform. You are apparently talking about every single platform. There are hundreds that kids use, or maybe thousands. You would need to have a government agency review content on every single one to determine if they are appropriate for over 21 or under 21. Which will be an enormously Herculean task since social media isn’t static. It is an ever-changing individual stream that is different for every person.
And there are lots of social-media type platforms that students use in fields like education. So you can’t just kick kids off the internet. A lot of online learning uses those platforms. So back to having the government evaluate each site and app for material and content you consider appropriate for over 21 and under 21. And then putting the entire over 21 portion of the internet behind an age-based paywall. Which will be made more difficult still because much of it isn’t even based in the US. Web sites based in say France aren’t going to be interested in your regulatory regime. So we end up will a walled off internet such as they have in China.
Your proposal suggests that you have no idea how kids even use the internet these days.