Your brain is "growing" until your early 20s, alcohol interferes with it, and while your mind is already mature enough at 18 to make sound decisions, chemistry doesn't give a fuck about your decisions or your feelings.
Well, most civilised countries actually teach children properly about alcohol etc.
Also, letting people try alcohol at a younger age is actually clever for a few reasons.
1. It removes a lot of the "cool" factor for drinking, making it just a regular/boring thing to them. 2. Because their taste etc isn't used to it, especially something like beer, it tastes horrible to them, which also helps put them off it. 3. When they're younger, it's easier to teach them about drinking in moderation etc.