For most of human history, consumption of milk was only possible for children, because all adults were lactose intolerant. Mutations in genes in Central Europeans allowed for lactose (which is in milk) consumption. However, milking of cattle may have started sooner in Asia. It started in many areas around the same time, which was when humans began to start being serious with agriculture. South America was the last place to get milk, having it come with the colonisation, because prior to that it was full of hunter-gatherer tribes, so they wouldn't stay in one place long enough to milk animals.
If you're looking for an answer like 'Jim Roberts, a farmer who escaped the insane asylum' then you doubt A) The time that milk started to be consumed, and B) What a species will do for survival. If the liquid from animals you're already farming is going to help sustain you, why the hell wouldn't you drink it?
