Given that porn can’t sexualize anyone under 18, she would have had to be at least 18 when she conceived her first child.
Since MILFs in porn are generally “my friend’s mom” or something generationally similar, ie the MILF is defined as such by someone the age of her child, said child and their peers must be at least 18 as well.
Therefore, the minimum age for a MILF character in a porn, legally and mathematically, should be 36
But you can’t depict a child in a porno, which is the specific case I’m speaking to here. I don’t disagree with you, generally