Someone under 20 with a child isn't really a MILF
How?
By definition, a MILF is short for "mother I'd like to fuck".
Ignoring minors, any legal age women who has a given birth can be classified as a MILF, even if they are under 20.
Yeah, people use it generally for older women primarily, because by current standards, women having kids at a (legal) young age is frowned upon, because having kids in general is now seen as backwards or pro-patriarchy.