Yes, OP's book's claims seem doubtful. I don't see many unaccompanied 6-year-olds in trains around Tokyo, but then elementary schools are typically within walk

Yes, OP's book's claims seem doubtful. I don't see many unaccompanied 6-year-olds in trains around Tokyo, but then elementary schools are typically within walking distance. It's fairly common to see an older sibling (say, 10~12 y.o.) taking care of a smaller kid. Anyway, maybe you can find some things that a typical Japanese x-year-old can do that an American x-year-old cannot. That doesn't mean they're "independent" from their parents, though. If anything, I'd say Japanese kids are more dependent on their parents that "Western" kids. Some 6-year-olds still bathe with their parents.