A 30-35 year old has a better chance of playing a near immortal sorceress who is 100 years old, or older, who lives in a medieval like period, fighting monsters, because of their life experience.
Yup, I'm sticking with that being a crazy complaint that she's too young.
No one on this planet has had any relevant life experience that would make them better at playing an immortal sorceress who lives in that world, so no one who has been alive longer has more experience or a better shot at playing that role. The only remotely valid argument is a 30-35 year old has more chance of being a better actress than a 22 year old as most people get better at their craft the older they get.
As for the last part, I never claimed casting always get it right, but the idea someone older would automatically have better chemistry is silly and I was pointing out that casting almost certainly tested them together to check they felt comfortable and sensible together. Of course they can get it wrong, but your argument is yet again flawed, because they could just as easily pick the wrong 35 year old woman who is either a bad actress or has no chemistry with Cavil.