Lactose (the sugar found in milk) is only tolerable in horses up to _ years of age.

The answer indicating that lactose is only tolerable in horses up to 3 years of age is correct due to the physiological changes that occur within a horse's digestive system as they mature. Young foals possess the enzyme lactase, which is necessary for the digestion of lactose. This enzyme allows them to efficiently break down milk, the primary source of nutrition during the early stages of life.

As horses grow and transition to solid feeds, the production of lactase typically decreases after weaning. By around the age of 3, most horses have a significantly reduced ability to digest lactose, leading to potential digestive issues if they consume milk or dairy products. This decline in lactase production is a normal part of their maturation process, reflecting a shift in diet from milk to fibrous plant material.

Understanding this process is vital for those involved in equine nutrition and care, as it impacts dietary choices and overall health management for horses as they grow.