The teenage body has a genetic code that is not yet fully set in, so any drinking or drug use can actually result in genetic damage. Such damage might not physically present itself until years or even decades later, but adult bodies can better withstand such substances, especially since adults will consume more responsibly on average
The reason this is true is because the brain is still developing at this stage of a person's life. In fact, your brain continues to develop until about the age of 25. Drinking before then can be detrimental to many of the connections that need to be made, and can cause stunting of intellect.
I think that drinking at an early age will cause more damage to your body and brain than it normally would drinking later in life. Before the age of 20, your body, as well as your brain, are still developing. The chemicals used in alcohol can cause damage to your body and can cause problems in developing. This could mean that you may not develop to your full potential.
Furthermore the brain is still developing up till around 25 and even then it merely tapers off. The brain is never done developing it just decreases in how much it changes. And I don't think it's a coincidence that this happens during a period of life where people tend to be making decisions about who they are and plotting courses into the future. Look at people who deviate from this and you'll probably find differences such as more settled brain development earlier or higher than average neuroplasticity still going on years later. And nothing wrong with either, we all figure ourselves out at our own pace. Studies already show when someone gets a new job as a taxi driver neurogenesis occurs in the hippocampus. Life experiences have a major influence in brain development. We should expect the environment to be influential even in common brain development because shared social norms and interaction would influence it as much if not more than genetics.
Drinking like many different activities can be part of this self-discovery process. Labeling any and all changes as "damage" is heavily biased. What about the shy socially awkward kid who doesn't really open up until he goes to college parties? Granted he could overdo it and become an alcoholic and then I'd say "damage" is a fair label. But he kid also keep in moderation, learn to open up a little and in that case I'd say alcohol definitely contributed to changes in how he develops but more likely for the better.
Drinking can do damage to your brain and body at any age. Drinking before the age of 20 wouldn't do any more or less (unless you were still growing.) It does, however, increase the chances of developing a habit of drinking - which would lead to a higher chance of damage.