Consuming milk post weaning
Milk is meant specifically for the very rapid body and organ growth that is required in the very initial stages of childhood. No other free living animal in nature, continues to consume milk once it has been weaned of it's mothers milk, since their bodies and organs no longer require the high growth food substance required in early ages of childhood. There is nothing different in the human body structure, that requires us to consume a food substance meant in nature exclusively for babies.
Consuming the milk of another animal
The mother's milk of any particular species is unique, based on the essential nutrients, fats, proteins, vitamins, hormones and enzymes that are required for the development of that particular animal baby. Humans are the only animal species that consumes the milk of another animal. Is it likely that the nutrient structure of cows milk (meant for an animal that weighs 8-20x our weight and with a significantly less developed brain) is similar to the nutrient structure required by the human body?
Nutrient composition of cow vs human milk
Even a brief study into the nutrient composition of cow's milk, clearly indicates that cow's milk provides a mix and type of nutrients that differ significantly from the nutrients required by our bodies. Cows milk has too much saturated fats, too little polyunsaturated fats, the wrong type of protein (caesin), harmful hormones, the wrong mix of vitamins, infectious particles and somatic cells. This nutrition structure (ideal for a baby calf, but wrong for humans - adult and child), is the key reason behind the large number of health complications and diseases associated with consuming milk.