A lot of people will remain lactase deficient. Yogurt and aged cheeses are more easily tolerated because some of the lactose has been converted to lactic acid. One rule of thumb is that the higher the fat content of the dairy product, the lower the lactose level. People who are still producing some lactase would then be able to eat a very rich ice cream but would be bothered by skim milk or ice milk. I found it at my local library.
Heavy cream is lower in lactose than light cream, which in turn is lower than whole milk.
Butter is higher in fat than any of these, and in turn is the lowest in lactose. It was mentioned that aged cheeses are lower in lactose than non-aged cheeses because the lactose had been broken down during the aging process. So, even though lactase deficiency and gluten intolerance can give the same symptoms, they are not caused by the same processes. Lactase is composed of two sugars. The problems arise when you are unable to break it into its two parts and absorb them.