5th grade seems like an appropriate time to start educating students about this as part of their health curriculum. That's the grade when they gave us the puberty talk in my old school district.
Some early bloomers absolutely start puberty during or before 5th grade.
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.
Yes they are two different things but they are related. Puberty can be devastating to a child who isn't comfortable with their body. In some cases puberty blockers are prescribed to delay the physical changes for children with gender disphoria. So educating gender identity along with puberty isn't a bad idea.
And yes, some children do start questioning their gender before they start puberty.
But this is all speculative. We don't know what the context for this teacher reading the book, but calling it inappropriate for their age is wrong.