Pediatrician

Milk is a good source of essential nutrients, including calcium; phosphorus; potassium; protein; riboflavin; niacin; and vitamins A, B12, and D. Vitamin D is actually added to cow's milk to help bones absorb calcium. All of these are crucial for children, and especially teens, whose adolescent skeletons grow very rapidly.

Issues surrounding obesity can make parents unsure about how much or what kind of milk they should give their children. I would be concerned about an overweight child drinking whole milk regularly. Rather, I recommend that children over age 2 and teens drink at least two to three 8-ounce servings of reduced-fat (2 percent) or low-fat (1 percent) milk each day.

Many parents are also worried that their children will develop a milk allergy, which usually appears during infancy in the form of a skin rash like eczema or atopic dermatitis. I don't recommend soy milk in place of cow's milk for infants because they may have a reaction to the protein in soy milk. I recommend a hydrolyzed formula instead. Most kids who are lactose intolerant can stomach some milk, or they can drink lactose-free milk. If a child can't or won't drink milk, consuming 6 to 8 ounces of yogurt or other low-fat dairy products, such as string cheese, can be a good alternative.
-Frank R. Greer, MD, professor of pediatrics, University of Wisconsin, Madison

Integrative practitioner

I recommend nonfat organic milk to try to minimize saturated fat, as well as exposure to growth hormones. Some people, particularly the Asian population, have a very hard time digesting milk because their bodies don't produce enough lactase, an enzyme that breaks down lactose in milk. For them, soy milk is a good alternative because it's fortified with similar vitamins and minerals as cow's milk and is nutritionally pretty equivalent. The main thing is to focus on an organic, vitamin-fortified soy milk without added sugar.

If someone can't digest soy very well, she could also try brown rice, almond, or hemp milks, which aren't quite as high in protein but are easily digestible. Or there's lactose-reduced milk, such as Lactaid, which has been exposed to an enzyme that breaks down lactose. Yogurts with active cultures are also typically not a problem because the bacteria digest the lactose.

If someone has a true dairy allergy, then she won't be able to tolerate milk whether or not it has lactose because she is allergic to one of the milk proteins — either whey or casein. In that case, that person would need to avoid all dairy, including yogurt and cheese. Again, soy is a good alternative.
-Cathy Garvey, RD, Scripps Center for Integrative Medicine, San Diego

Geriatric nurse

I don't usually recommend milk for older adults because as they age, they often lose lactase, an enzyme in the small intestine needed to break down lactose in milk. Too much milk or other dairy can cause stomach upset, constipation, or diarrhea. People over age 65 who have any lactose intolerance should not consume any milk. They do, however, need to get at least 600 IU vitamin D and 1,200 mg calcium every day to prevent or manage osteoporosis. Vitamin D can also be obtained through sun exposure, but older people often have to limit their time in the sun to reduce their risk of skin cancer, thus putting themselves at risk for a vitamin D deficiency.

Calcium depletion can result from a variety of factors, including chronic illness and certain medications. Older adults who maintain a balanced diet can get calcium, vitamin D, protein, B vitamins, and other nutrients by eating green, leafy vegetables and small amounts of other dairy products like cheese or yogurt. Usually 6 to 8 ounces of whole or low-fat milk is fine as long as it doesn't pose any digestion or allergy problems. If someone is allergic or intolerant to dairy, I recommend taking a calcium supplement — unless that person has kidney stones, which supplements can sometimes encourage.
-Neva Crogan, PhD, associate professor, University of Connecticut School of Nursing, Storrs, Connecticut