Why are first teeth called 'milk teeth' ?

I know that the first teeth coming through can cause problems at times, because
they can feel sharp, and some mothers wean from the breast because of this.

But presumably to nurse *to two years or beyond* means nursing with these first
teeth, at least with the sharp front ones, because they usually come through
sometime in the first year or so.

I have a question: is it more usual for a girl to wean at an earlier age than a
boy ?

Helena Boutal