There's so many questions I think that our MC is The Reincarnation of the saint sara in the reason why I come to that conclusion is because in chapter one she had a dream and in that dream she saw that world that she was summon to and I think since there's a Time difference in between dimensions by the time she was summoned to that world the original Saint would have been the same age as the Sara from Earth
Reincarnation does not mean gender bender reincarnation means you are born as someone else if you retain your memories from your former life you're not the same person that you once were when you died you can't stack the ages on top of each other
This is not Ranma 1/2 kampfer or birdy the mighty which are stories where the main character is transformed into a girl by magic or item
Do we get some Ranma 1/2 going on here