I've always thought the trope was based on the idea of stunted development. Kids are heavily encouraged to drink milk, so films making a point of adults drinking it are indicating that there's some part of growing up that they've missed.
I've always thought the trope was based on the idea of stunted development. Kids are heavily encouraged to drink milk, so films making a point of adults drinking it are indicating that there's some part of growing up that they've missed.