I think it depends on the individual. To some, milf could be your actual age. To others, it could be adopting a certain look or maturity in the way you dress. But to me, milf is more of a vibe than actual age or look. My example right now of this is Kenzie Love. She just turned 30 literally like a month or 2 (or 3) ago and most of her career are milf scenes just because she has a serious milf vibe about her. See also: Melanie Hicks.
To each their own of course, but I think milf vibes usurp all else, and our girl Lauren here is definitely 100% a milf since she's been like 27 lol.