Explanation

Milk sugar (lactose) is digested by the enzyme lactase, which splits it into glucose and galactose. Most adults progressively lose lactase production after weaning, causing lactose intolerance. An elderly person who can still digest milk must therefore still be producing enough lactase. Cellulase digests cellulose, amylase digests starch, and pectase acts on pectin, so none of them explains milk digestion.