Why doesn't lactose in Parmigiano Reggiano keep decreasing as it ages from 12 to 36 months?

Answer: Early bacteria turn lactose into acid before aging really starts.

Early bacteria turn lactose into acid before aging really starts. ✓ — Right: the main lactose change happens early, not during long aging. Lactic bacteria ferment lactose into lactic acid in the first days after production, so most of it is gone before the wheel reaches 12 months. Aging from 12 to 36 months still changes texture and aroma, but it is not the main lactose-removal event.

The long aging period slowly breaks down the remaining lactose. — No: the important lactose change does not wait for long aging. The chemistry is fermentation, where microbes transform the milk sugar near the beginning. Many aged-food changes are slow, while this one happens before the cheese has the old-wheel character people notice.

Water loss during aging washes the lactose out of the wheel. — No: lactose is not mainly removed by water loss or washing during aging. The key step is microbial conversion into lactic acid during the early production period. Long maturation can change flavor and texture dramatically, but it is not the main reason the milk sugar disappears.