BlackVulcan
Male
There have been under-aged m0thers since the early days of humanity. Just to be technical.Obviously, she needs to be a mom! But I believe when this was created, it was more common for women to become mothers at a certain age. The mom of your school friend couldn't be a 20-year-old girl!
But the standard pervy meaning of MILF more or less applies to the mature moms of about 35 to 45. And that, too, is pretty obvious.