How about the same number that lets you vote, and decide where you live, or get drafted, and get married without parental consent, and get medical care without parental consent?
Voting, moving out, getting medical care, and marriage generally don't negatively impact your body and brain.
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).
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.
I didn't mean to imply that it was any less arbitrary (the arbitrariness of the numbers is disputed in other threads), only that I view the feds setting that number as overstepping.