Pretty much it's a legal definition, and isn't really reflective on the person. However because it is a legal definition they (people in general) us it as a basic point of delineation and cutoff.
You're in for a few more rude awakenings about the 21-25 bit. In many states in the US you cannot even rent a car unless you're 25, pay an extra fee, or have a secondary driver that is 25.
In many states you cannot rent a car until you're at least 21, or rent a hotel room until you are.
When I turned 18 I could buy lotto tickets and gamble, but not drink. A year later when I was 19 they had changed the law so that I couldn't buy lotto tickets or gamble. They didn't even grandfather people in.
The 18 bit is basically because you have a lot of highschool kids that turn 18 when they haven't graduated yet, and in this country that typically translates directly into a lack of respect for alcohol consumption and how it can harm you. It is in fact a poison that you're consuming. But, people generally felt that it was too easy for someone to turn 18 and start providing their under-age cohorts with alcohol.
When I turned 18 my mother pulled me aside and said, "I was able to drink when I was your age, and I trust you to not drink and dive if you do make the choice to drink. But if I catch you smoking I will kick your ****ing ass."
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."