I found a rather interesting article on the aging body's tolerance of milk. Here it is:
I was rather shocked to see that 75% of adults around the world cannot ingest a lot of milk. Maybe this just went over my head while reading chapter 5, but after seeing that lactase-persistence varies with genes, it never occurred to me that babies around the world still drink milk, and may stop after weaning. I was surprised that lactose-intolerance was more common than lactase-persistence. It appears that of the many adults who develop some degree of lactose-intolerance, the majority could probably digest milk as babies, but lost that ability "as early as 3, some at 20, some in their 60's and some never at all."
This raises a question about the age at which one loses that ability and what other factors affect this. If we were to perform a long-term study on adults and mark the age at which they lose the ability to digest milk, what correlations would we find with ages? Perhaps ancestry in populations where lactase-persistence is common?
I could be wrong, but perhaps maybe lactase persistence then, does not simply mean being more able to digest milk. Maybe populations of people with higher rates of lactase persistence still commonly lose the ability, just with a higher average age. If this is true, it would significantly adjust the definition of lactase-persistence.