Getting trashed is developmentally harmful to pre-18 brains, getting pregnant or knocking someone up or getting a disease is not helpful, and there are easier ways for most people to learn than to make horrible and even irreparable mistakes very early. Also, bad habits often survive into adulthood. This isn't where anyone should be forced to fit into the world until after they have a real chance to decide.
If a kid learns to have just one drink, don't be a junkie, use protection and waste time only judiciously by 18, they will be able to learn more from higher education or career than if they learned it's good to act like a mindlessly hedonistic animal with no concept of the future.
You can spend your first 40 years sticking your tongue into electrical sockets, this is technically some form of learning, but it's not a great way to have a good chance in life. There is all the time in the world to waste and act stupid, after you have the basic bearings of adulthood.
Yeah, but... If you are the one teenager who decides it is too risky to jump off the cliff into the river... Enjoy sitting on your own while everyone else has fun - enjoy never getting invited to the river again - enjoy sitting at home playing videogames as the braver boys wander off with into the grass with all the cute girls.
Teenage impulsivity and risk-tasking are reproductively optimal strategies (the actual rates of death and disability from these activities are usually low anyway) even if you like to look down your nose at them. And people who engage in those activities often end up doing just fine in later life, as well-adjusted, popular, confident adults, with a history of sexual success.
Maybe they're not optimal strategies for being a high-performing time-efficient adult, but so what? Natural selection only cares about babes and babies.