system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did I become lactose intolerant at 18?

I've recently discovered that my stomach pain has been caused by lactose consumption. I've drank atleast 1 glass of milk a day for as long as I can remember. Why am I now not able to eat/drink it? Shouldn't my body be used to it?
gpt
It's actually quite common. Babies are very rarely lactose intolerant, because lactose is a part of breast milk so being able to digest it properly is pretty important for babies. However, as we get older, we historically no longer needed to digest lactose, because we stop breast feeding after a year or 2, so a lot of adults lose that ability to digest it. It's actually considered to be a useful mutation, especially amongst Europeans, who have the lowest levels of lactose intolerance, to be able to still digest it into adulthood, because cow milk was a common part of the European diet.

tl;dr: Babies need to digest lactose for breast milk but people older than that don't, so a lot of people lose the ability.