It seems to me there are two kinds of MILFs. Some men like the idea of a woman who is already involved with a ch1ld. They may see her as a chance for an extramarital affair, or as a divorcee, or simply a single mom. This sort of MILF keeps the age low; technically you could have a 25-year-old MILF without demanding that she have some dysfunctional biography.
However, in analyzing the term MILF, I think it's fair to remember: who coined it? I first picked it up from the movie "American Pie," and I think it fits in with the whole "You mom is hot!" topic, as set to music by the Fountains of Wayne in "Stacy's Mom," technically a variation on that old infamous hit "Hot For Teacher." To keep all parties involved legal and prudent, I would push to a minimum of 40.
The business gets a little more complicated by women aging differently. It's typical for women to lie about their age, or just avoid mentioning it, but sometimes you meet a woman, hear her age, and respond "You've got to be kidding!"