I think Mab and Lea have to be cast in their thirties, simply because of the length of filming and the need to remain youthful. even at two books per season/year, it'll take 12 years to tell the story.
Maeve and Aurora were supposed to look barely legal, but that can be fudged.
I've actually wanted one person to play Maeve, Aurora, and Sarissa, and let costumes, acting skill, and makeup turn them into different people. see Tatiana Maslani in orphan black for how that could be done.