Based on Ed Greenwood's twitter post, it is, at least it is what it work in FR setting. You have to be born with a capacity for magic to wield any kind of magic, even for divine caster, 1/2 or 1/3 caster. And only small number of them become wizard.I know a lot of people who assume wizards have to be born with a capacity for magic that not everyone has.
Edit: And the chance a human born with that capacity is very low except place like Halruaa where generations of caster boots the chance A LOT higher.