I think the triplets all look like they are late teen (18+) women with different body types, as is Alita.
Now - for the sake of the story, all 4 are written as being somewhat naive with little practical knowledge of men and, given what happened to Gracie, that makes sense re: the triplets (I don't know enough of Alita's backstory to give specifics for her).
The only female in the game who is, IMO, Not 18 is Norah and I simply choose to treat her like the child I see her as.
You get to choose the path of the MC so if you think of the triplets as too young - did you choose only the pure path or did you try multiple paths?
I think one of Radiant's challenges is that it offers 3 paths that will, over time, tell increasingly different stories so future chapters will, by necessity, become more complex if it remains true to showing the evolution of each path.
If you find that you cannot overcome the 'look' of a number of the young ladies/women in this story, then it would seem like it is not for you.
There are Many other options on F95 to choose from that have more mature looking female characters.
I wish you luck in your search for AVNs you can enjoy.
Cheers!!