I tend to ignore people dunking on Americans because, let's face it, a lot of the criticism is warranted. But this seems completely out of left field. I've never heard this stereotype before.

We don't infantilize people in their early 20's. American's don't view 23 year olds as "naive kids." We tend to view 18 and year olds that way but by the time you hit 22 you are definitely an adult. A lot of us move away from home for college, spend a year or two in the dorms and then get a private apartment. Finances permitting, people tend to choose not moving back in with their parents after graduation. Also keep in mind that a significant chunk of Americans don't go to college and enter the workforce/military/whatever at 18.

What kind of dorms are you talking about? That sounds more like a boarding school than a college dorm. The only rules involved fire hazards (no candles) and no illegal substances. I don't know anyone who has had a dorm experience even remotely like that. Maybe at extremely religious private colleges? But those would be an exception to the rule.

I know the drinking age being 21 is seen as dumb by most of the world, and a lot of Americans feel that way too. Most Americans drink before 21 anyway.

I don't agree necessarily with arbitrary maturity lines like drinking at 18/21, but suggesting people might be thought of as adults at 14 is madness. Most kids aren't finished going through puberty at that age and it's different for everyone (by like 5-6 years potentially). I think 18 is the "arbitrary" age for most things because 99% of kids have finished puberty at that age and we aren't in a rush to get those 14-18 year old working in factories.

The whole "capitalism delayed adulthood to 18" arguement doesn't make sense to me considering very capitalist mills and sweatshops have historically used child labour throughout history.

Also, very important point most people ignore, the human brain doesn't finish developing until around 25.