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
I think their logic is more that one of the big archetypal MILFs is “your friend’s hot mom”
Your friend is probably about the same age as you, so if you’re old enough to appear in a porn, your friend is probably is too (and has to be if they appear in the porn at all, even in a non-sexual role)
And the mother would have had to have had your friend at 18 because they want to avoid implying anything about a minor having sex (I don’t know if that last part would actually hold any water, that seems like a bit of a stretch)
This is precisely the context I was speaking to