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.