In most cultures with a majority of lactose tolerant people, there's a lot of lactose in everyday food, so people missing the mutation usually never complete their weaning before long in adulthood, when for some reason they “stop or drastically reduce [their] intake”.

Unless you know for sure that you have the mutation, stopping drinking milk or consuming dairies at some point in your life comes with a risk of not being able to drink milk ever again in your life. (I don't mean that going vegan is bad or anything, but I wish people knew it can come with a bigger commitment that what you may expect)