[AndrewC]

How about mutations in the proximal region to the lactase gene that allows people to continue drinking milk into adulthood? Not an advantage?

So what is the explanation for the "disadvantage" to be the normal condition for all of the mammalian class?

From wikipedia:(I got lazy)

The normal mammalian condition is for the young of a species to experience reduced lactase production at the end of the weaning period

[allmendream]

Easy. Do most mammals ever get a chance to drink milk other than in infancy? Why waste resources making an enzyme (lactase) that would never be of any use?

It took domestication of milk giving animals before a mutation of this proximal region to the lactase gene would be an advantage; and apparently it arose at least twice independently, once among Northern Europeans, and again among cattle herding Africans (they have different mutations).