What is a good thesis statement for an argument of the drinking age should not be lowered to 18?

At the age of 18 you are not very responsible yet, it is dangerous for you and everyone around you.

Hope this helps

Nonsense! Clearly the drinking age should be lowered to 18. If a person is old enough to vote, and has the mental capacity to decide who should make the country's laws; if he is old enough to be drafted into the army to be trained to kill and to die in his country's service; if he is old enough to be held mentally competent to stand trial and possibly face the death penalty, then he must surely be old enough to decide whether or not he wants to consume a bottle of beer. Plus, it's American beer, which means it only slightly more alcoholic than water anyway. :-)

You could also point out that making something illegal only makes it more desirable, and that binge drinking of the sort that leads to most young adults' irresponsible behavior, is far more common in the States than in countries that allow them to drink at an early age. [Hey, that gives you something to research. Imagine writing a research paper that references empirical studies].

You could likewise point out that the reason most adults drink is because they believe the benefits outweigh the drawbacks, and that there is no reason why this should be true at 21 and not at 20.

And if anyone should be foolish enough to object that people shouldn't drink because they should be getting an education, you could naturally respond that learning to balance socializing and work is a part of a good education.