[chandra does nothing for her shivering. suffer.]

Technically, any human can cast magic, provided their blood hasn't been too diluted with sura heritage. But yes, it depends on their birth attributes and, above all, their Divine affinity. Although having a high affinity doesn't mean anything if you lack the skill and mentality to make the most of it, those born with a low affinity will never have an aptitude for magic no matter how hard they practice.