Youth who drink before the age of 15 are times more likely to develop alcohol dependence than if they wait until they are 21 years old.

According to the National instituto of Alcohol Abuse and Alcoholism (Or NIH), people who start drinking before the age of 15 are 4 times more likely to meet the criteria for alcohol dependence at some point in their lives, than if they started drinking afther that the brain is fully developed (and this happens around the 20s)

So it is safe to say that:

Youth who drink before the age of 15 are four times more likely to develop alcohol dependence than if they wait until they are 21 years old.