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
In my opinion, MILFs make more sense to be in the middle adulthood, not too young and not too old, between the mid-thirties and late-fifties. While a young woman could be a mother, she is too young to be a MILF.