No. Now, as ever, it meant a MOTHER. And being a MOTHER isn't age restricted. I know my MOTHER was one when she pushed out my older siblings in her 20's. Me as well. Almost as if you don't start being a mother when the kids hit their teens. It starts when you push em out, regardless of age. Rest of you might wanna wrap you minds around the fact you can be a mother without being MATURE.
Yes, we understand the term and the biology of the process of having a child. And yes you are correct, that any woman who has ever had a child qualifies for the phrase. Just as any woman who may have had kids very yooung (18 or sooner) and their daughter or son was a parent early as well, could be a GILF at the young age of 36, i don't think many women would like to be labelled with that abbreviation when they might still think MILF was flatterring (each person is different of course).
However... The point of the original meaning of the phrase was targetted at women who were the "soccer mom" age category. Why? Because a good portion of porn at the time was heavily focused toward women in their 20's or early 30's, and not as much on the older women in their mid/late 30's and eapecially 40's. As my former comment apparently hit a nerve with you, you may want to research that a bit to verify.