- Moms I'd like to fuck. This term probably originated in high school when high school boys would talk about their friend's moms. While technically an 18 year old 'mom' could be a MILF, its generally considered that a MILF is a mom with a high school child. However, many sites will stretch that down to about any girl who is no longer teen... so about 25 or 28.
The term MILF was made famous in the movie American Pie, and has become a very common term in the porn world. This has in many cases just become a replacement for the generic term amateur. While MILF is often used in relation to women who are obviously over 40 (as was originally intended), there is now almost certainly an overlap between MILF and mature. Mature generally is accepted to refer to women over 40, while MILF now includes women down into their 20s.