At some point you pick a number, it will always seem somewhat arbitrary.
Alcohol, weed, cigarettes, etc. all do some kind of damage to your body. If anything we should raise cigarettes to 21. And yes, even weed, especially smoking weed, does damage to your body, even if it's massively less than the other two I said.
(And getting drafted... well that should just go away all together).
Guess it shows how much smoking cigarettes has been reduced in society (or how out of touch I am...).
For sugar, what do you actually ban? Added sugar? There's plenty of sugar in fruits, especially when you juice them. How do you enforce this? Seems like a nightmare to figure out.
Caffeine is interesting. It's actually physically addictive. I can't find any studies on long term effects of caffeine in children, though.
And, of course, there's exactly 0 support to do any of that, which can't be ignored. Probably a better use of everyone's time to improve health education.
Regulate medical marijuana like any other non-scheduled drug and leave cultural decisions concerning recreational use to the states, where it belongs. Use federal funding stipulations as leverage when needed.