The culprit may be fast movement of food through the digestive system that’s caused by other foods the lactose is eaten with, or by a genetic predisposition. Digestion can also slow down or speed up with age, which is why people notice more digestive disturbances as they get older. Grand says that many people with low lactase can still digest some milk because bacteria in their large intestines take over the task of digesting lactose. For those who feel they truly can’t digest it, he recommends calcium and vitamin D supplements to prevent deficiencies.
https://www.bostonglobe.com/lifestyle/health-wellness/2013/09/22/health-answers-why-some-people-become-lactose-intolerant-they-age/kVFLdq2fL784gybRiIfkvM/story.html
Briefly, for some reason your body does not produce lactase, or does not produce enough lactase. Lactase is an enzyme that allows your body to break down lactose (a type of sugar found in milk). There can be different causes for lactose intolerance. See Lactose intolerance for more information.
https://www.quora.com/Why-are-some-people-lactose-intolerant
Most people with lactose intolerance can tolerate some amount of lactose in their diet and do not need to avoid milk or milk products completely. Avoiding milk and milk products altogether may cause people to take in less calcium and vitamin D than they need.
https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance
Not having enough lactase in the body is the cause of lactose intolerance. Primary lactose intolerance. This type develops in people who were once able to fully digest lactose. It is the most common type. Almost all infants make enough lactase to fully digest lactose found in human milk and infant formulas.
https://www.nichd.nih.gov/health/topics/lactose/conditioninfo/Pages/causes.aspx
Not having enough lactase in the body is the cause of lactose intolerance. The names for the three types of lactose intolerance describe why a person may not have enough lactase: Primary lactose intolerance. This type develops in people who were once able to fully digest lactose. It is the most common type.
https://www.nichd.nih.gov/health/topics/lactose/conditioninfo/Pages/causes.aspx
While lactose intolerance is a digestive system disorder, a milk allergy is a reaction by the body’s immune system to one or more milk proteins. An allergic reaction to milk can be life threatening even if the person eats or drinks only a small amount of milk or milk product.
https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance
There is another theory, however, that it’s not the lactose but the fat in cheese that makes certain varieties difficult to digest. The curdling process converts a good deal of milk’s lactose into lactic acid, while whey, the leftover liquid, washes much of the remaining lactose away.
https://www.chowhound.com/food-news/53922/why-can-lactose-intolerant-people-eat-some-cheeses-and-not-others/#!
Why Can Lactose-Intolerant People Eat Some Cheeses and Not Others? Given our seemingly insatiable obsession with foods like mac 'n' cheese, grilled cheese sandwiches, and mac 'n' cheese–stuffed grilled cheese sandwiches, it’s easy to forget that there’s a good chunk of people out there who can’t eat dairy products.
https://www.chowhound.com/food-news/53922/why-can-lactose-intolerant-people-eat-some-cheeses-and-not-others/#!
In fact, those people are the norm: Roughly 75 percent of the world’s population has difficulty digesting lactose, the primary sugar in milk. If you’re lactose intolerant, many cheeses may leave you feeling bloated and uncomfortable—or worse.
https://www.chowhound.com/food-news/53922/why-can-lactose-intolerant-people-eat-some-cheeses-and-not-others/#!
People have lactose intolerance when lactase deficiency and lactose malabsorption cause digestive symptoms. Most people with lactose intolerance can eat or drink some amount of lactose without having digestive symptoms. Individuals vary in the amount of lactose they can tolerate.
https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance
what causes some people to be lactose intolerant
Not having enough lactase in the body causes some people to be lactose intolerant. Not having enough lactase in the body causes some people to be lactose intolerant.  