Well, most of us probably first heard the term in the film American Pie (although it was apparently around earlier). So yeah, the archetype is the hot mom of your teenage buddy, which means she is likely in her 30s or later. The actress who played Stifler's mom was in her late 30s at the time.
Before the MILF we had the Mrs. Robinson! Anne Bancroft was 36 when she played that role. Am I dating myself a bit here?