First Amendment doesn't cover things that cause direct harm or speech to minors. In this case, you can draw a straight line between social media usage and an increase in teen suicides, and keeping them on it for long periods is detrimental to their overall mental health especially when they don't have fully developed impulse control or are not legally expected to. You're not allowed to advertise gambling products, alcohol and tobacco to minors under the First Amendment. You can't intentionally show porn to a minor without going to jail in many cases. (18 U.S. Code § 1470 - Transfer of obscene material to minors)

You want to get adults addicted and express yourself to them, so be it, but the legal distinction on limitations on what you can express to minors is well documented.