In today's age, MILF's can be quite young due to the influx of young mothers. This has spawned a new elite kind of MILF, the Legitimilf, whom is usually older than 30 but remains very attractive. This term has come about due to there being to many young mothers who are naturally attractive. Legitimilf's must work to stay attractive due to being slightly older, many refer to the need to work at attractiveness as reaching the "downhill slope". By maintaining attractiveness on the "downhill slope" the Legitimilf's have earnt their elite status.
When the "downhill slope" becomes to steep the woman has reached her "Milf-by-date"
Erica: "OMG did you hear Stacey's just had a baby?"
James: "Wow she's like a total MILF now!"
Keith: "Dude have you seen HER mum though, now that my friends is a Legitimilf."
Hilary Duff is a boring MILF. Reese Witherspoon and Kate Hudson are Legitimilf's. Stifler's Mom has reached her Milf-by-date.