Are we all Lactose intolerant
Adults of all mammals should be.
I don't know why you are raising this question, but as I understand it, YES. Children under 5 years are okay. Then we outgrow our ability to digest lactose.
Yes. Most of us at least. If forced, body still can produce lactase (the enzyme needed to digest lactose) in some cases.
However, lactose is not the only thing in milk that can be prohibitive. Casein, the milk protein is hard to digest and can cross compromised blood-brain barrier.
Then there is composition of saturated fats in cow's milk that is suitable for calves, but not humans.
I am not even talking the actual treatment of cows, what they are being fed and injected...
...off my soapbox for now.