Mathematical maturity should be well defined if you are using it as a prerequisite. Mathematicians, of all people, should be good at coming up with such precise definitions.
Some people are better with math than others. Undergrad math majors thought the major was pretty easy. Many of us in engineering majors--who weren't that bad at math (it was a good school)--could no more have graduated with a degree in math (or physics) than have flapped our arms and flown.
