You can ban it from appstores and ban traffic that looks like nostr traffic at the router or ban access to nostr relay servers. If people use tor on a device that lets you install your own apps, yes you can get around it. Banning the mobile apps would block most users. In this case, appstores do know what age the user is, and could not let users under 18 install social media apps, including nostr apps. A teen could either just change their birthday on their device or use a vpn to appear to be in another state. In terms of computers, tor would be the best way around being blocked. 

It doesn’t matter, these laws will never be held up in the courts. It’s pure political theater.