Porn, cigarettes, and alcohol are just vices and not in the same category as internet access to social media sites. Exposure to social media can be good or bad depending on content whereas exposure to vice has no redeeming value whatsoever. And in any event the government has no business sticking its nose in matters that are the duty and prerogative of parents.
The same argument you’re making for social media can be used for anything you call a vice.
Shouldn’t some things be reserved for people who (supposedly) are old enough to understand what they’re consuming/seeing/reading/ingesting?