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
*She, and they are very rarely implied in porn unless it’s in the context of finishing/boarding/imaginary 18+ young adult school.
Pornographers who are above board in any way at all wouldn’t dress a set to imply there is a minor present