Ability to digest lactose as an adult is something that only recently (~10,000years) evolved in humans, coincident with the act of domesticating cattle. East Asian populations generally have high levels of Lactose intolerance (up to 98% in some groups), people of northern European descent generally do not have this problem (only 5% are lactose intolerant).
Half of Thai adults show lactose intolerance/inability to digest according to the study below, but other sources say 90%+.

Lactose intolerance in Thai adults. [Pubmed]
http://en.wikipedia.org/wiki/Lactase_persistence