It is not normal for humans to drink milk after 2 years old – and from another animal on top of that fact. Researchers have found the gene that allows predominantly Eastern Europeans who have inherited distant dairy farming relatives genes to continue to produce the enzyme lactase after being weened from their mothers. Lactase is the enzyme that digests the sugar in milk and when absent, allows this sugar to ferment in the colon making one quite gassy. So it’s not that some people are lactose intolerant, it’s that the abnormality are the people who can tolerate milk.

From USA Today:

A group at University College London has shown that the mutation actually appeared about 7,500 years ago in dairy farmers who lived in a region between the central Balkans and central Europe, in what was known as the Funnel Beaker culture.