Is it true that most people are lactose intolerant?

Yes, in world view: Babies typically are not lactose intolerant--or they could drink mother's milk; but in cultures where dairy is not commonly consumed, lactase, the enzyme that breaks down lactose, dimimishes markedly over the first 4 years. In cultures where dairy became a common part of the diet, a genetic modification arose to prevent this drop. Also, ongoing use of milk often keeps lactase production going.

Varies: There are subtle differences in lactose intolerance. Some have it from birth, some acquire it as they get older (allowing mothers milk to work in infancy), some have it transiently after any diarrhea. The data I've read suggest 1/4 of the adult population in this country are lactose intolerant.