Although the literal expansion of MILF would include mothers of any age, even high school t33nag3rs who just gave birth, when people say or write "MILF" they normally mean somebody who is in the age range where they could unsurprisingly be expected to have children in their late teens (i.e. the scenario from American Pie, the movie which popularized "MILF"). So that would be a woman in her mid 30s up to about 50.
That was my whole point - using a term MILF without knowing exactly what you are saying simply shows the lack of awareness of the user. The "M" of MILF stands for
"Mother". So, the person in question needs to have offspring.*
A more appropriate term for the mid 30s+ age group that would fit is the recently more popular
"Cougar". If someone says that there is no predetermined assumption of progeny.
So that would imply a term such as CILF.
However I feel that terms like this say more about the user and less about the subject.
I like VAMP - "Very Attractive Mature Person"* 'Mother' is more fitting for a woman who has actually reproduced, the act of being motherly can be applied to any 'Mom'. Those are two terms that have also blurred over the years but for many other reasons.