Lactase... fickle bastard allele

Dear Mrs. Intarweb-Science;

Prior to the domestication of cattle about 8,000 years ago, humans had access to lactose only when they were young and nursing. As a result, the gene that allows us to digest lactose (called lactase) typically gets shut off during childhood, and stays off for the rest of human life.

I hear people say sometimes that they have become lactose intolerant.

This is apparently what happens after nursing, if you don't have the magic "lactase allele" of the white people who bred cows.

Oh; wait... or you could have this new lactase allele that the Tanzanians & Kenyans have recently developed... in fact; you probably recently starved to death if you didn't, because everyone who does have this allele has a huge amount of genetic similarity.

OH oH; no wait... there are other people in the universe who don't have either, and yet aren't lactose intolerant. (Science isn't just the truth of white folks?? gee!)

So; I'm a believer in evolution, and I think it's really exciting that there might be a really strong example of the process of selection happening in humans. But here's where I'm confused. If an ability/disability to eat something is not so much based on what is going on in your gut (I know folks who stopped eating milk for two years and then developed lactose intolerance), how does that mesh with the genetic description? I have trouble understanding how genes suddenly "turn off" -- or is it just an aging thing?

How have these discoveries not disproved the relationship between lactose and a billion different factors? If having one of many lactase alleles can keep your lactose intolerance from "turning on" after nursing, how does this happen at a random time later in one's life? And what the hell does "turning off/turning on" mean? If a bunch of different alleles can cause lactose tolerance to remain active, why do they think it's a gene and not the state of one's intestines? How do some people become lactose tolerant again by re-populating their guts with the right enzymes? Can you turn the gene back once it has flipped?