What makes a woman a MILF?

MILF is an acronym for "mommy I'd like to fuck", and I have noticed the term is often applied to women that are older than about 40 years of age.

Does a woman have to me a mom in order to qualify as a milf, or can any older yet sexually attractive woman be a milf?

If she has to have kids in order to qualify as milf, does that mean a 23 y/o woman who's become a mom can be considered a milf too?