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.
Anyone can go to pornhub and see a landing page full of highly explicit sexual images without any kind of age verification whatsoever. How is this legal, if what you say is true? Is it just that they don't know it's a minor on the other side of the screen, so they can say it's not "intentional"?
Pornhub home page has a very clear warning before you get to anything explicit:
This is an adult website
This website contains age-restricted materials including nudity and explicit depictions of sexual activity. By entering, you affirm that you are at least 18 years of age or the age of majority in the jurisdiction you are accessing the website from and you consent to viewing sexually explicit content.
I am 18 or older - Enter
I am under 18 - Exit
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.