None. Now, if we were talking about purchasing age and drinking age as separate entities, there'd be a good argument for having some minimal limit (18 perhaps as a good round figure seen elsewhere in western cultural norms). But we're not. And as purely a dividing age between people who can never have alcohol ever, and get tossed in correctional facilities for doing so, and people who can drink until they kill themselves and no one cares, such a thing runs counter not only to our supposed values but to common sense.
Here's the thing. As great as science is for most things, and as nice as having a basic guideline for how alcohol distributes through the body via measuring weight, muscle mass, and gender is, it's just that. A guideline. How large an effect alcohol has on a person varies by liver function, enzyme production, brain chemistry, as well as variables of addictability, self-control and all round social function. This all adds up to what we generally consider 'tolerance' As such, there is no set number on how drunk a person is until you reach certain extremes of motor impairment and thusforth.
Because drinking is, across all western cultures, a social activity, an expected function at many socializing events, it is important for a person to recognize drunkenness in themselves and have a working concept of how much alcohol it takes to trigger drunken reactions for them. And it's not something you discover magically once you're 'an adult' and have imparted to you through divine intervention. You will be a dangerous drinker until you have learned your own limits and what drunkenness is for you. It's a skill, acquired knowledge learned via direct experience, and as in all social skills, putting it off until some nebulous 'adulthood' is just asking for dysfunction.
Though certainly alcohol can have debilitating effects on development, and must be carefully monitored during earliest life. But it's the sort of thing any good mentor-figure should know and impart along with everything else to the learner.