Allergies come from proteins. The body's immune systems treat proteins that leak through the intestines and into the bloodstream the same way they treat truly dangerous invaders, like bacteria. However, there is more than one immune response system in the body. Some of these create what are called hypersensitivities, and these can produce symptoms much like LI, especially in young children. For a much fuller discussion, see my Lactose Intolerance versus Milk Allergy page.
Lactase is the last intestinal enzyme to develop and is present in the smallest quantities.
Hardly surprising then to learn that it is also the first to go. Generally speaking, those populations with the highest percentage of LI among adults will also lose their lactase at the youngest age. A few may even start losing it in their first year of life. In others, lactase production may fall very, very gradually throughout life so that symptoms from milk will not be noticed until adulthood. A few people have first reported lactose intolerance in their seventies.