A problem of logic

Parents are not even preferring the policy which (necessarily) best protects their kids. Parents are preferring the policy which gives them a slightly better feeling of being in control of their kids, whether or not they are.

This is the biggest problem with most of our most paternalistic policies and opposition to a few which actually do have some positive effects but which surrender that illusion of control that parents want to maintain.

So the following things are made illegal or are restricted to discourage children's use
1) Narcotic drugs
2) Alcohol
3) Tobacco
4) various contraceptive devices or prescriptions

Some of these are dangerous, or at least mildly so for children and teens. 3 of them are potentially habit forming and carry at least modest to severe health risks for adults (though the order in which they carry those risks is not related to their legal status). The last carries the stigma of ameliorating an apparently undesirable act (sexual activity).

Of these, the last resembles something else which (some) parents resist
1) Vaccinations.

Basically proven and safe vaccinations over many decades of use and contraception resemble seat belts in that their use reduces, sometimes dramatically, the risks of normal human life and behavior. We get sick, and it would be best not to get sick and transmit that illness to others. Strangling those diseases from having many sources of incubation so that they can spread is a huge positive externality effect. The only trouble is that the only point of contact parents see is a needle going into their child. They do not see invasive biological agents that are potentially far more lethal and dangerous and hence they wish to exercise control over what they can see. The same logic applies to sexual acts and condoms or birth control, or to the use of mind-altering substances. We presume that if these things are banned and rendered invisible, like bacteria or viruses, they will not exist and we won't have to deal with our children potentially using them or engaging in such activities.

The trouble is that the reality is that we have children who in fact do deal with these "invisible" dangers. On average they are having sex by 17, well before we think they would if our home rules against sex, laws of sexual consent, and under-age access to sexual devices like condoms or birth control would apply. It is of course absurd then to pretend that they will avoid sexual contacts simply because they can't get to a condom or to a pill or a shot or anything at all which might give us some measure of actual control over the results of those encounters. But the illusion of control allows us to pretend otherwise. They are experimenting with mind-altering substances, only it's easier to come by if there are no legal regulations governing sale to minors. If it's a black market only as with narcotics, there are no legal regulations. If our object was really to keep drugs out of the hands of our children, we wouldn't be sitting around making them completely illegal. But the illusion of control tells us that if they were legal for adults, more kids would use them or want to and it would be harder to discourage them from using, and so they remain illegal.

What we should ask is what it is we actually want. What we should want is not "more control for parents" over... other people's kids, conceived by passing laws. I'm pretty sure what we actually should want as "parents" is fewer kids using narcotics or drinking or smoking or having sex, at least without responsibly dealing with the consequences of any of those behaviors. And presumably we want fewer kids getting preventable dread diseases (from STDs to polio to MMR). Since the reality is that kids will be kids, and many of them will do some, if not all of these risky behaviors, and that this happens despite our "well-intended" laws, we should probably look into changing how we approach these subjects with laws. Ideally we would act to counsel them actively on how to do these things responsibly, just as we might teach our teens how to drive a car. Our object is at best to put seat belts there in case they do silly things as young people, just as they will speed or drive recklessly.