They also talk about “Asians” not having the gene to digest milk. This is true of most south Asians, but most north Asians have lived on the stuff for millenia. Mongols and all that.
The "mutation" (Yes, we can call it that, a mutation imbedded in the matrices of mutant milk drinkers) started somewhere, and spread.
It only takes one copy to give you the gift, which will continue on into adulthood provided you continue to consume milk and milk byproducts.
Worth noting many East Asian people who believe they can drink milk (without also taking lactose pills) consume vast quantities of kim chee and saurkraut (same thing, different continent). That's where they get their lactase enzymes. BTW, cabbage grows well at high latitudes. Also worth noting East Asians who don't eat their kim chee discover the gift of projectile vomiting if their little problem begins at the pyloric valve! I have seen cookies tossed 10 meters over a large campfire.
I tell them "Read the labels" and eat no cheese aged less than 1 year.