This is a very common excuse used by ageists. They say that they were teenagers at some point, and that they were out of control and needed prohibitions. An easy response to that argument is that the fact that you were an idiot does not prove that all teens are idiots. But I don't even have to stop there. If we look at the environment created by the prohibition, it becomes obvious that if teens are irresponsible, it's caused by the prohibition.
I'll use my experience with the drinking age as an example. I started drinking about two months after turning 18. If there was no drinking age, I probably would have started at 16 or 17. Supporters of age restrictions would say that since the drinking age delayed my onset of drinking by 2 years, it was effective. It seems like that would be true, but there are other side effects that are frequently ignored.
The reason I estimated 16 or 17 is that that was the age that I became curious about alcohol. The drinking age may have stopped me from drinking, but it did not stop me from being curious about alcohol (I don't see how it can). Without the drinking age, I would have been able to go to a party with anyone, or go to a local bar. Because of the drinking age, I had to try to fit in with the popular group in high school and early college in order to have access to alcohol, no matter how big of jerks they were. The constant rejection made my life hell, and many other youths feel the same way.
Another problem was that I felt like I had to drink every chance I got. There are two reasons for this. One is the Forbidden Fruit, which means that I felt like the law was taking away my liberty, so I had to rebel against it. I felt that if I didn't drink, it wouldn't be my decision. The other is the Last Call, which means that I felt like I wouldn't get another chance anytime soon, so I had to take each and every one.
My experience was as an unpopular person trying to get in the popular group so I can gain access to alcohol. Even for popular people, it ain't much different. For them, it's about trying to stay in the popular group so they can keep access to alcohol. Some young women even risk being around rapists just so they can have access to the beverage of their choice. If they could buy their own alcohol, it would be easier for them to dump these jerks and find better friends.
I actually had access to alcohol once when I was 17 at a graduation party. I didn't drink because I was driving. I was smart enough to not drive, and the other partygoers were smart enough to make sure I didn't drive. But what if I wasn't smart enough? It was my first time having access to alcohol. What if I took the chance and drank? Some people who aren't as smart would have taken the chance. If there was no drinking age, there would have been no need to take the chance. It would have been possible to plan on drinking and therefore been possible to get a designated driver, ride a bike, or walk. When people don't plan on drinking, they're more likely to drive drunk.
There's this bar I like to go to that's about 3 miles from where I live at college. I don't have a car there, so I ride a bike. Two weeks in a row I couldn't go because of the snow. I was fine with that, because I could just wait until next week. But what if I was underage, either if I was younger or if the drinking age was higher, and the party was at a friend's house 3 miles away? I would have been much more tempted to try to ride my bike in the snow.
I never had an overdose, because I mostly knew my limits. But what if I did and I needed medical attention? Would my friends have called for help, or would they have been too afraid? Would I have died in the time it took them to get over the panic? Would I have really deserved to die if I could have been saved?
Some teens, like some adults, are irresponsible, but it's very likely caused by the prohibitionist environment. Those who are responsible don't need prohibition, and those who are irresponsible would either ignore the prohibition or be placed in a more dangerous environment. The prohibitionist propaganda-filled education makes their environment even more dangerous. The most irresponsible ones, those who are willing to hurt others, would be unaffected by prohibition because there are already laws against those dangerous behaviors, and those willing to break those laws would also very likely disobey the drinking age. The positive effects of the drinking age are therefore not very significant.
The drinking age appears to have many side effects, even some that people don't even think about. Unpopular teens try desperately to fit in with the popular group, and popular teens try desperately to stay in the popular group. Underage partygoers are afraid to report emergencies like rapes or overdoses. Underage drinkers are tempted to drink every chance they get, and the chances they get aren't always the safest. Since they often don't know when they will have access to alcohol, it is much harder to plan ahead and get a safe ride home. There are also other side effects, like the violation of liberty, the invasion of privacy, and the segregation. Are all of those side effects really worth it, just to delay the onset of drinking by only a few years?