@Magaso I mean, she wouldn't be able to work out much as a baby, and her muscles wouldn't be developed enough to even hold a sword for idk how long, but probably a while. Meanwhile, that 13 year old has probably been training for at least 5 whole years, and has naturally developed muscles and mass on top of that.
I mean, can a 5 year old actually train their body to that extent? Bodily control can be discounted, but I highly doubt the muscle mass and total mass of a trained 5 year old can stand up to the power of a trained 13 year old's muscle mass and total mass.
Anyway, if the author made her like 10, it would have been much more believable IMO