
The term MILF in the literal sense used to encompass even nubile teenage girls who happened to be moms. But now? It looks like the requisites for classifying as a MILF don't even include being mid 30's or over, like it usually was in porn, but a combination of being able to pass off as one with "cougar makeup" and the reverse of the "teen paradigm" (where petite, flat-chested women get marketed as teens even at 26 yo like Gia Paige for example; the reverse being buxom, less-petite 20-somethings being marketed as MILF's).
Adding to the point above: we could have a situation these days where a natural 21 yo porn starlet, who is marketed as a teen, stops and comes back a few months later with big fake boobs, big fake lips and other facial plastic jobs, and voila! She's suddenly marketed as a MILF. She just needs to use makeup that makes her pass off as a cougar.