The bottom line is that parents, and the law, are doing the right thing by keeping most addictive substances away from children before their brains have physically matured.
Before the age of between 18-23, the brain is far more flexible than at maturity. And it will adapt to addictive substances much easier, and more, the more of those substances it is exposed to. Once the brain is adapted to those substances, addiction is far easier, and breaking addiction is far more difficult.
Importantly, there seems to be some crossover, because one addictive drug can lead to having a “addictive personality”, leading to other addictive drugs. This is the “gateway” problem of ‘soft drugs’ leading to ‘hard drugs’.
Children should also be discouraged from having positive associations with addictive substances. Such associations motivate people to consume such substances from reasons than the substances themselves. For example, first tobacco use in a calm and not comfortable situation focuses on the fact that tobacco tastes awful. But in a party atmosphere, with food and drink and other diversions, the awful taste can be ignored in favor of the positive associations. Associating tobacco use with fun.
So the bottom line is that such substances should be infrequent and minimized in childhood, and efforts should be made to prevent positive associations with them.
This is why AA and some Cognitive Behavioral Therapy are required for alcoholics to keep them disciplined and on track. As an alcoholic, it takes a lot of work to think through the consequences of my actions. Even if I’m abstinent.