There are a lot of things in adaptions that don't make sense in the context of the source... this is why they are called adaptions. You are setting yourself up for disappointment if you are expecting a perfect ripped from the pages representation of The Witcher. If they age up characters, the story will be adapted to fit the change. It's irrelevant if it doesn't make sense based on the books because the plot and characters will be tailored to fit the needs of the show. If Ciri has to be aged up to 20 or older, she will be and it could be for any number of reasons (like not being able to find a capable actress in her early teens that can handle the content of her story arc and/or said capable young actress' parents allow their child to be subject to the key moments required by their child)
Now I am not saying this is what is going to happen, my point is merely you can't expect a perfect adaption. Keep in mind, the core demographic that will watch this show, knows NOTHING about the source allowing the showrunners even more freedom to portray the characters how ever they want. Also, whos to say that the show starts at the earliest point of the books?
Man, I really dont understand why you want so much 20 year old ciri or why you want to die in the hill of defending lousy loosely adapatations.
Im not setting myself for dissapointed because im not thinking its going to be 1:1 the books (maybe the short stories are shown, or maybe not, as much as I would love for the first episodes to be that, but I already know to not wait for them to be there, and if they are, amazing, but if not, somewhat comprehensible), but im 100% sure that they will start the Ciri story from when she is a kid because she being young is a very important part for the story to make sense with her evolution and Geralt's, taking that part would make zero sense. If the series writer changes that aspect, then they are fucking up the source material so much that why use it. You start entering the Dragon Ball movie adapatation terriotory right there.
Yes, an adaptation can go the Harry Potter route or the Dragon Ball shitty movie route, were everything is completely invented, but the DB type of adapatation happens MUCH less than the other type.
An adpatation that makes older Ciri in the backend of the series to look like the videogame counterpart doesnt destroy the narrative of the story, I wouldnt mind it, even if changed from the books, the soul is still there. An adapatation that even tries to make the Ciri story from the beggining as an adult woman fucks up with everything, as much as making the witcher take place in a modern setting college with CW cast.
Your excuse about the general audience not knowing the source material so they can do whatever they want to do... even the people that have only played the games know about young Ciri becuase the 3rd game starts with a flashback of her at that age training, so thats already is a big "wtf are they trying to do here" even for the bigger game audience. Not even talking about Polish people that are probably going to be the stricter audience of this series and could reject it completely like the old Geralt polish TV series.
That and they already said it was based in the books story. They are not going to get away with that kind of stuff if they try, and I dont even think Spakowski is going to even let them try in the first place.
About the "like not being able to find a capable actress in her early teens that can handle the content of her story arc and/or said capable young actress' parents allow their child to be subject to the key moments required by their child" what happens with her at the start of the books is not that far from other adventure fantasy epics, and nothing that a child couldn't do. If the script needs a child, it needs a child, even for movies like Leon the Professional, no matter how harsh the subject is (apart from, theres always techniques to not show the harsher things to kid actors during filming).