I could write a book here about pharmacology and alcohol in the human body. But since this is my first post I'll spare you.
Children under 18 YOA should not consume alcoholic beverages on a regular bases. Yes there are immediate consequiences (that laws prohibit) but it is the possible long term affects that need to be thought about. Most children are not fully developed at 18 YOA. Regular consumption of alcoholic beverages can disrupt organ development. This is a long term affect, as it my not rear it's ulgy head till you start a family or have a serious illness and can't (or slowly) re-cope from.
All States' DUI laws state "under the influence of" not drunk or can't drive.
After a drink or two do you feel the affects of driniking? Like you face is a little flush, you feel a little at ease, you feel courageous, just a slight buzz. Guess what you are under the influence of alcohol. Sure you can walk, and even drive but should you?
FYI each regular mixed drink or 8 oz glass of wine and 12 oz beer. each have approx. 1/2 oz of alcohol. Regardless of your size most humans can only eliminate 1/3 of an oz per hour. You do the math.