When many of us were kids, we used to eat cereal and milk every day without any trouble, and “lactose intolerance” was unknown. But as we grew up, I think a lot of us probably stopped eating cereal. I know I did.
So now, whenever I try to have some milk and cereal I get bloated, get gas, runny nose, etc. No fun. Fortunately I can still handle cheese just fine.
I found a trick to be able to eat cereal and milk without having problems though. The secret is all-bran cereal. The fiber does something but basically I found that if I had all-bran, no problems at all.
I imagine that if you want to build tolerance quickly with minimal trouble, just have a bowl of all-bran cereal once a day for awhile. The body will probably start producing more enzymes to help break down dairy products.