So my understanding is that normally the enzymes to process lactose are available in newborns.

If we follow the design Mother Nature intended, as soon as we start eating solid foods we stop drinking milk and or body stops producing those enzymes making us effectively lactose intolerant.

But since humans do drink milk the body continues to produce those enzymes.

Some people still lose that that ability and then there are also ones who never had them. That makes them lactose intolerant.

Also being lactose intolerant doesn't kill you, you just can't break it up, so you will have upset stomach, fart etc.

I believe there is a lactose free milk for newborns for those scenarios.

My understanding is that there's lactose in all milk so probably it applies too.

Also besides being lactose intolerant, you can also be allergic to milk, which is much more serious, but that can vary on kind of a milk.