I'm not sure I get your arguement here

Are you saying that we should let children smoke and just tax it because its better for their liberty and freedoms?

Or are you saying we should just tax social media for adults but banning it for kids is ok

We do that here; heavy tax sigarettes (and booze): both dropped like a lead balloon. So yes, tax it for everyone. Kids cannot pay for sigarettes and most adults don't want to (most vapers I know do it because it costs far less; that should be taxed more too imho). If browsing insta/tiktok costs an euro per hour, let's see how many still do it; I'd say they go bankrupt in a few months. Apparently it was never that interesting.