Great question, I wonder that too. Looks like "milf" has taken on a new meaning...

I use the term Milf when I can determine by scientific means that a woman has borne children. I agree too many people throw that word around too easily.

Slacker has been doing that for months. Still doesn't understand.

Seems to me Slacker views all females as MILFs.

It's because the average male consumer of pr0n is a fuckwit (in this, I include myself).
See also, blokes saying they like "amateur porn" when there's obviously a lighting rig, a cameraman, a website watermark logo. It's almost like the commodofication of human sexuality rots your brain, or something...