While various Marvel Cinematic Universe entires, The Irishman and other high-profile movies have worked digital magic to make its actors look younger (to varying degrees of success), Miller would rather avoid coming anywhere close to the uncanny valley effect, which is reasonable.
We’ll just have to wait and see who’s brought aboard to play the younger Furiosa, but George Miller evidently feels comfortable with having an actress in her 20s involved rather than playing around with digital de-aging.