if he's doing 17-21 year old Furiosa and what made her join and eventually rise up to the rank of Imperator in Immortan Joe's crew then yeah it wouldn't make any sense to have Theron who is 44 IRL, especially since the movie won't start shooting for another year or two.
The more interesting question is if they jump back 20-25 years, how does time coincide with the apocalypse and Mad Max? I don't really know my Max lore, but he was around when civilization collapsed in the first Max so it might be half of a "apocalypse starts" movie, this time from Furiosa's perspective.
i'm not a fan of prequels, we already know she won't die, any friends/kids she has in the movie WILL die, and she loses her arm, so a lot of mystery is gone. Still, George Miller has gotten a lot of good will and trust from me with Fury Road so hopefully he can deliver. His directing credits are nuts, he did Babe 2 in 98, Happy Feet 1 & 2 a few years later, Mad Max Fury Road 4 years later, and i don't think he's worked on anything else since.