Right, but adults are assumed to be somewhat more responsible for themselves. This is why we don't let kids (legally) smoke or drink, but we do let adults do so. We expect that adults can, in general, say no, and that children are less able to do so.

But it's not absolute. Some drugs are illegal for adults as well, for example. Why? Because they're too addicting.

So are Instagram and Youtube just nicotine, or are they heroin?