It's definitely not clear-cut in Witcherverse, just like it's not clear-cut how exactly magic works. My understanding that mages (lets call them that - eaier to type, and that includes both male and female) are humans, elves, or mix-bloods between those two races - because I don't recall any mentions of dwarven/halfling/gnomish mages - who are born with an inclination/talent for magic. It's undeveloped but it has to be there to begin with (if you recall Yennefer tests Ciri, then tells her she didn't even need to test because Ciri was able to detect the magic of Yennefer's necklace easily). So then later this ability gets developed through training, studying, practicing, etc. It also seems like different mages have different affinities - healing (Marti), illusion (Fringilla), and so on. Doesn't mean they can't do other magic, just that some of it comes easier than the rest.

I think it's sort of implied that developing their magic powers, or prolonged use of it, or both, does change their physiology to an extent - but not nearly as much as mutations do for witchers. It doesn't really seem that those changes are beneficial, rather they are consequences of the strain the use of magical power puts on the body. Thus, sterility (in most cases).