Many mammals become ill if they drink milk as adults even though they could consume it as babies. what causes this digestive issue?

As mammals grow, they stop producing lactase.

Explanation:

Mammals consume their mother's milk during the first stage of life. Milk has a sugar called lactose. Lactose is a disaccharide formed by the union of one molecule of glucose and another of galactose. To absorb it our digestive system releases an enzyme (lactase) that breaks the union between galactose and glucose. As mammals grow, they stop producing this enzyme, which makes lactose indigestible. As a result, ingesting milk may cause  diarrhea, gas and bloating.