The younger you start using an addictive substance, the more likely you are to become addicted.
Putting an age limit on drugs use is a matter of public health, and protecting people who are not mature enough to understand the consequences of their acts.
Some people take more time than others to reach maturity, so I agree that setting an arbitrary age is not ideal, but it makes some sense.
If it were that simple, a country like the Netherlands which has a legal drinking age of 16 (for ABV<20%) should have way more problems with alcohol abuse and addiction than a sensible country such as the USA, which has the drinking age no less than five years later, so significantly less people in the US should suffer alcohol addiction, right?
Incidentally, we don't get to take a driver's license test until we're 18, so that's kind of reversed. Care to guess who has more problems with driving under the influence?
Your comparison doesn't make any sense. There are other factors, notably cultural, involved. For example, the praise of individualism in the US (which may encourage reckless behavior), and the limited social security net (poverty increases the risk of substance abuse).
When you control for other factors, the pattern I described emerge. See:
Of course children have to be protected. That's the whole point of education, gradually removing barriers while you teach them (or let them learn) how to get along by themselves. Whether or not the society should have a word on the topic is open for debate. I'm for, to some extent, as you probably guessed.
Putting an age limit on drugs use is a matter of public health, and protecting people who are not mature enough to understand the consequences of their acts.
Some people take more time than others to reach maturity, so I agree that setting an arbitrary age is not ideal, but it makes some sense.