Once again not all parents are fools. They try their best but frankly young kids and teens don't always listen, been there done that I got lots of 't-shirts and other paraphenalia for souvegniers of that age and immortality complex. Frankly I think they should raise the age of many things to 21. *shrugs* Just because a few are mature enough to handle things does not mean the majority.
As far as the age to serve their country, well that is quite a different matter. Before you are given a m-16 and sent to war there are months of preparations, (I went through these preparations but never finished so didn't get enlisted) that help prepare these boys and girls. They have the initial visits from recruiting officers, they have to get into shape to pass the physicals, they have to sign 100000 forms, and depending what they are enlisting as, such as I was enlisting for defense foriegn languages and had to take a DLAB test on top of ASPHAB and quite a few other tests. Its not a spurr of the moment lets just go down to the corner market and fly to Iraq for the night.
As far as voting, well hell most people dont really understand politics as much as they like to profess to, so meh 18 year old has just as valid opinion as a 35 year old.
You wouldn't let your 95 year old grandmother with alzhiemers disease sign papers for the deed to her house or other properties so no need to allow reckless teenagers buy as much alcohol as much as they want..at least now their parents have to buy for them or they have to go around and find friends to, who could go to jail and I know in LA they could get their driving license revoked as a punishment for buying alcohol for minors, or get fake ID's which just puts another step in between and makes alcohol consumption a bit less spurr of the moment idea.
Its a buffer, parents should be the protective wall yes but as we have seen in zoos everywhere sometimes things get around protective walls. Kids will always find ways to illegally do stuff..do drugs, buy guns, drink, but its not really a good excuse to just legalize everything.
Added: I actually think that people with a military ID should be able to buy alcohol at 18..the Military kind of makes people grow up and face real life, and get over the immortality complex.