IMO, milk is for babies. It has a high sugar content, which in excess (of which the definition is individualized), may contribute to health problems in adults. People may also be drinking milk in combination with high-sugar cereals or additives (like chocolate milk). It might be OK for an active 2 year old, but for an older adult, not so much. Sugar is not good.
That being said, people should eat and drink whatever they want.
I always considered milk a children's drink. I know some adults drink it but not sure how healthy it is for them. Beer or hard cider is definitely better health-wise.
Not that dairy is all bad. Cheese, yogurt, and cream are good products made from milk that provide health benefits for adults.