For me, it's the fact that you're legally an adult at 18. You can go out and get a tattoo or whatever and have it permanently etched into your skin, but you can't drink. It makes no sense to me.
I agree that 18 is a good age, I just think it should be done slowly. Like a phase in. I mean, I have no idea how to go about that practically, but just dropping it three years seems like there could be room for a whole lot of things to go wrong.
And just to point out a small error in your argument, there are several states that allow minors to get tattoos provided that you have parental permission and most states allow tattoo artists to tattoo family members regardless of age. Since my sister and brother-in-law are artists, I could have gotten my first tattoo when I was 10 and they got married and it would have been perfectly legal. I'm not looking to start an argument, I'm just saying that there are some reasons why your point isn't the greatest comparison to make in this situation.
Around here, kids get drunk all the time. UNH is a fairly big party school. I don't want to be around that lifestyle more than I have to, so I live in the Chemical/Substance Free Dorm. Most of us that are here have our reasons for not doing drugs or drinking (for many of us, it's watching family members and friends be destroyed by alcohol) but there are a few who do go out and drink/party. But mostly they respect that they're the minority in the dorm and if they're going to go out, they make arrangements to stay elsewhere.
One of my best friends went to his first party for homecoming or something. He was a freshman and this was his first college drinking experience. He came back completely wasted (pretty close to alcohol poisoning). When he woke up (after several hours of just plain being passed out) he said he couldn't remember much other than that he felt extremely pressured to go beyond what he felt were his limits. He's since sworn off drinking for at least the remainder of his college career because he had a really rough time making up for the two days of classes he missed.
Days like 4/20 and Cinco de Mayo are hell on campus. So much so that some professors cancel class because they know that the campus gets rowdy and not many people are going to bother showing up to class. I'm just glad that St. Patrick's Day fell over Spring Break. I heard the stories of what happened to some people who stayed on campus (two girls were raped and three more people ended up in the hospital with alcohol poisoning) and let's just say I'm not in a hurry to experience that holiday at school.
Halloween was just as bad. We spent the night outside watching all the drunk girls try to walk in the heels that came with their "Sexy Whatever" costumes. Now that is quality entertainment.