Man it was wild when my GI doc gave me the low-down on that. Like most everything in metabolic science its a “grey subject.”
Mammals naturally lose the ability to produce lactase as they wean off mother’s milk. However, humans, particularly Europeans and some areas of Africa have consumed dairy for long enough that we do maintain limited lactase production if it is introduced shortly after weaning. There is evidence in some areas of western Europe specifically, where life long production of lactase does appear to have evolved.
But for the majority of the world, yeah, they day we started weaning was the day we stopped being lactose tolerant.