Well, if one can see a Cesarean (sp?) scar or an episiotomy (when they enlarge the birth canal/vag by making a cut so the baby can come out) scar on the woman, that's a pretty clear sign that she's a MOM, at least.
So, technically speaking, that could be a girl who's not even of legal age for sex or porn.
I think people think of mothers who have kids that are at least a few years old, and maybe more than one. When I see/hear "MILF" I envision a woman at least in her late 30s, but probably no older than 50...
It's mostly just a matter of opinion, though, just because the "ILF" part is a matter of taste. If a woman isn't a mother then she can't really be a MILF.