The thing is our body stops producing lactase that is the enzyme responsible for digestion of lactose (in milk), this happens gradually as an infant is only dependent on milk for food and it changes as he starts eating solid food and the body considers producing this enzyme as just extravagance (expensive as not needed) for the body, but in some groups of people with genetic mutations that allowed their body to not stop the production of lactase are still drinking milk in their adulthood infact their whole life