According to Wikipedia, under protein, drinking cows milk makes the bones less dense. Americans eat plenty of cheese yet have high rates of osteoporosis. Here is the reason why. Animals foods are very high in protein. Eating a lot of these high protein animal foods causes the body to become too acid.
This leads to many diseases. The body tries to neutralize this acid by taking calcium out of the bones. The calcium bonds with the acid and it is then urinated out. So Americans eat plenty of calcium but they have a lot of calcium in their urine. It is better to get calcium from foods like oranges or leafy greens.
Of course vitamin D and other nutrients are necessary for good bone density. Also it is important to do weight bearing exercises to stress the bones. Weight lifting does a great job with this. Carrying a baby or groceries also helps.