Human brain development is correlated with behavioral development, and the brain can fully mature as late as the mid-20s. Teenagers, by and large, are literally operating with a mental deficiency relative to adults. Compound that with a lack of experience and knowledge and you get bad decisions. Further combine the mix with substances which interfere with normal mental activity and potentially lethal devices? Why is that a good idea?
Again, it's not about being "stupid", it's about an average individual's ability to make reasoned, intelligent decisions. Having an underdeveloped brain reduces that ability, as does impairment by way of alcohol and/or a range of drugs. The consequences of a bad decision are much worse when the result is driving home while plastered, going cliff diving after doing shots, throwing stuff in front of trains, &c. versus falling off a porch or saying something socially awkward. Teenagers have a hard enough time making good choices as it is.
(sorry to bring this up again, but I feel I still have unmade points)
My counter is that while you can point and say "this person isn't as capable as they will be in X years" its ridiculous to say "this person isn't as capable as that person because that person is older" because there isn't any such thing as an "average" individual. EVERY SINGLE PERSON is "literally operating with a mental deficiency" in relation to everyone else, because our brains develop DIFFERENTLY.
Just because someone has a MORE developed brain doesn't mean it developed in a worthwhile direction that lends itself to understanding consequences.
There are tons of adults who have horrendous decision making, go to a trailer park, pick an adult there and compare them with a teen in the Honor Roll. Unfair comparison? Maybe, but that's the comparison that we've been constantly shown in the opposite direction when people want to talk about how dumb kids are. "Here's a successful businessman in the one percent versus this wannabe gangster getting drunk and...I dunno lighting his farts on fire or something stupid"
In short, there are PLENTY of people who shouldn't be allowed to make their own decisions, but it has very little to do with how long they've been on the planet.