Nearly all infants benefit from milk, but most adults cannot properly digest milk. If an elderly person can still digest milk, his body is producing enough of which enzyme?

Milk contains the sugar lactose, which is broken down by the enzyme lactase into glucose and galactose. Infants produce ample lactase, but in most adults the production of lactase declines, leading to lactose intolerance. An adult who can still digest milk normally is producing sufficient lactase. Cellulase digests cellulose, amylase digests starch, and pectase acts on pectin — none of these are involved in lactose digestion.