I think you might be underestimating the level of control that an average parent, especially a working parent, has over a teenage kid. Short of taking away devices, it's tough, especially if they're going through a phase of doing precisely the opposite of what you recommend / demand.

I'm not saying that parents don't have any responsibility, but it's about practicalities. If a teenager can easily buy smokes or alcohol, many will, no matter what the parents say. If you make the goods harder to buy, usage drops. So, shops / software vendors do have some responsibility for societal outcomes.

In a libertarian utopia, anything goes, but kids are... weird in that they often try to push the boundaries of their autonomy without always knowing the risk, and it's in our collective best interest not to let them go too far.