I think one difference is when you have 18 as the legal drinking age, a significant chunk of kids turn 18 while still in year 12. They could then legally buy it and then (illegally) on-sell it to their school friends who weren't of legal age yet.
In some cases parents would even buy alcohol for their teenage children. Parents supplying other drugs to their children is not totally unheard of but far less common.
> I have some stoner friends from high school who were really smart and gave up in college and now work mediocre dead end jobs and live in really low cost of living areas.
> They're happy. They're far from dead. But that potential they had is completely gone at this point.
What entitles you to their potential?
People may want to change the world, or may just want an entry level job that keeps them fed and housed while spending their time on drugs/videogames/what-the-hell-they-damn-want-its-not-my-problem
If really smart people decide that working in construction and smoking weed is their call, who am I to say otherwise?