COPPA is a federal law that made it more difficult for social media sites to monetize and track US users under 13 years old, so Meta properties won't let you make an account, if you don't lie about your age, if you're under 13.
And lying about your age is easy to do, so Meta knows that they have young children using their site, and they take all of that into consideration in terms of growth and getting kids hooked, exactly like tobacco companies were/are doing with cigarettes and now vaping.