They have to be middle aged or else the acronym is pointless. As you point out, women can have babies starting around age 14, so by that logic a woman could be both a "Milf" and under 18. Over at Wiktionary they define a Milf as a woman at least 35 years old with grown children. In American Pie the character sleeps with a woman who is old enough to be his own mother, right? That's how I've always thought about it. Back in high school, everyone had a buddy or two with a mom that was hot.
That said, I find it offensive when porn sites categorize women as "Milfs" when they are clearly under 30. It's degrading to the models, and frankly I don't know why people go for it. Maybe it's just the flip side of the "I'm only 18 and this is my first shoot" nonsense you run into with so much porn. My guess is that Milf porn is actually kind of lucrative and that is why producers are so eager to mislabel it. For me personally, I've got a little bit of a thing for middle-aged women, and I can't be alone with all the Milf porn out there, but it angers me when I'm looking for older women and I find a gallery or a video with a girl who could pass for 23. Sure ages are extremely flexible in porn, but if she can pass for 23, she's not what I'm looking for and she's not going to do it for me.
