Can you develop lactose intolerance with age? or is it possible to have had it as a kid, have it go dormant for a while then come back in your 20s?

Increases with age: Lactose intolerance increases with age/ if you had it earlier in life, it could have been as a result of a diarrheal episode with a temporary lactose intolerance.

Lactose intolerance.: ....Increases with age, yes. It can also be a temporary side effect of gastrointestinal illnesses.

Many possibilities: Lactose is the sugar in breast milk, so most have the enzyme to digest (tolerate) it at least in infancy. There are a few that are deficient in infancy, and many of these have it transiently after bad diarrhea. By adulthood as many as 1/4 will be enzyme deficient (intolerant) .Many of these have gradually selected a lactose free diet over time & may not self identify as intolerant.