I grew up in America, and that view of milk as a children's drink was pretty common in my Scandinavian/Germanic heritage in the US.
When we went to restaurants, children ordered milk to drink, but an adult never would.
Milk was fine to put in your coffee or tea, but not to drink a cup of it. Milk was fine to use with cereal or to cook with, but not to drink plain.
I do not doubt your perspective at all; however, there are definitely at least parts of America where milk is not for adults to drink. My recent ancestors were dairy farmers in the upper midwest. I cannot think of a single time my parents or grandparents drank milk straight up. I won't say it never happened. It could just be my faulty memory. But if beer or coffee or juice was available, pretty much every adult would drink those. Even juice was pretty iffy. Juice for breakfast maybe. Any other time, and it was a child's drink as well.
Obviously, this is just one anecdote. Don't take it for data.
Having also lived in Germany and now Sweden, I will say that I believe milk is not often drunk plain by adults in either of these places.
My experience is the same. Children drank milk with meals, while adults drank coffee, tea, or water and juice at breakfast. I got my first taste of coffee at about 7 while eating lunch with my father in the fields.
Perhaps someone in the restaurant business could say how often adults order milk compared with children.
In the US the only milk my grandparents would drink was buttermilk (they called it "milk") that they made by culturing overnight on the kitchen counter. They called ordinary milk "sweet milk".
When we went to restaurants, children ordered milk to drink, but an adult never would.
Milk was fine to put in your coffee or tea, but not to drink a cup of it. Milk was fine to use with cereal or to cook with, but not to drink plain.
I do not doubt your perspective at all; however, there are definitely at least parts of America where milk is not for adults to drink. My recent ancestors were dairy farmers in the upper midwest. I cannot think of a single time my parents or grandparents drank milk straight up. I won't say it never happened. It could just be my faulty memory. But if beer or coffee or juice was available, pretty much every adult would drink those. Even juice was pretty iffy. Juice for breakfast maybe. Any other time, and it was a child's drink as well.
Obviously, this is just one anecdote. Don't take it for data.
Having also lived in Germany and now Sweden, I will say that I believe milk is not often drunk plain by adults in either of these places.