Milk contains two types of Protein: whey (20%) and casein (80%).
Research shows that a mixture of slow and fast burning protein is superior for muscle building. The mixture of whey and casein present in milk enables lean body mass gain.
Also, both whey and casein are high quality proteins containing all the essential amino acids in amounts sufficient to support the multiple roles of protein in the body.
Summing up: Protein is a key factor in building and maintaining muscle mass throughout life and consuming milk in one form or other as part of a balanced diet will help keep you strong.