To understand why aged cheeses have little to no lactose, we need to examine the cheese-making process. Dr. Rachel Johnson, Ph.D., RD, a professor of nutrition at the University of Vermont, explains, "During the cheese-making process, the milk is separated into curds (solids) and whey (liquid). Most of the lactose remains in the whey, which is drained off." As cheese ages, any remaining lactose continues to break down. This process, known as fermentation, is carried out by bacteria that consume the lactose and convert it into lactic acid. As cheese ages, any remaining lactose continues to break down, resulting in lower lactose levels over time.

Source: Gardner's Wisconsin Cheese