It’s not exactly „becoming lactose intolerant“, it’s „getting weaned“.
Every mammal in the animal kingdom gets raised on mother’s milk for some time after birth and at some point loses the capability to digest it. Most people know they should not feed adult cats with milk, unless it’s a specially prepared lactose free „cat milk“
I first heard that some 20 years ago - before going vegan myself - from a work colleague who was „lactose intolerant“.
I asked him why, and he told me that at age 12 or so, he had a sports injury - he‘s an avid soccer player - where he broke a leg and the doctor told him he should not consume dairy for a few months to improve the bone healing. When he tried to consume dairy again after his injury had healed, he found it no longer agreed with him.
We often had spirited discussions whether cappucchino is an abomination or not - he’s Italian - and he always conceded that for a German/Austrian person like myself it’s ok to consume, but anybody serious about enjoying coffee certainly should stick to Espresso…
Humans (to be precise: humans in the Western world) are the only species of mammal who keep consuming dairy after adolescense and keeping the capability to digest dairy through this consumption up to an old age.
But then, that’s not a problem at all - there are so many great plant drinks available now.
Sidenote: We used to understand that „Asians are lactose intolerant“, and it dismays me very much to now see the numerous advertisements whenever I watch TV in Asia nowadays where Western dairy companies try to convince Asian mothers to „give the best of Western food“ to their infants (dairy milk, bone broth and similar) , so they can „become stronger and smarter“