I think the common theory is that lactase production is stopped during early adolescence in mammals as a sort of safeguard to not compete with younger siblings

I think the common theory is that lactase production is stopped during early adolescence in mammals as a sort of safeguard to not compete with younger siblings for milk (when you're old enough you can get other food, while your younger siblings depend on milk). Some human subgroups (e.g. Europeans) are different, as they mutated to produce lactase even in adulthood (an evolutionary advantage in farming societies).