I like this interpretation of history. I guess that means the Wright brothers were drinking Red Bull?
More seriously, they drank milk because they were pastoralists with a lot of it, and because it’s something you can consume. Hell, they were even lactose intolerant; if you happen to be European or West African all the better. Not sure where this “it’s for kids” idea comes from, maybe it’s some kind of American cultural thing. (If you want to be really bio-deterministic, it’s for babies, older children are in the same group as adults)