Some do, some don't. There are milk cultures, Europeans, and others, Native American, Chinese, where mother's milk is the last, for it does not exist in the culture.
As long as you drink milk, fine, stop and the ability to digest it vanishes. African cattle cultures can drink milk, but not the non cattle adults.
I have problems with wheat, but not rye and oats.
Cheerios are oats, and with goats milk yougurt, and fruit, fine.
Cow milk is another thing.
Thousands of years of adapting to a local diet, then this mix and mess.
If there is nothing else, and starting with small amounts, we do adapt, but don't drink milk for a few years, then drink a lot, you might just have problems.
It is about the same as engines, there are a lot of fuel choices, jets will burn anything, they are set for kerosene, but will burn gas, diesel, 151 rum, but performance changes, by products, and the fuel/air mixture has to change, often drasticly, or the engine melts, or clogs, or explodes.
We are self tuning, and on any one fuel adjust, but it takes time.
A bland and constant diet is best.