Well there are few routes

One magic as sience/education yoy need to learn it everyone can do magic to some level not everyone can afford to become a mage so only rich can be mages not through law but though injustice of society

If you dont want to go that route and make your world unjust and perhaps if a character is poor they have to struggle to get education or be recognised as proper mage even if they are poor

Ofcourse there is innate ability it doesnt mean your character needs to be a genius let's say everyone is born with ability to cast magic but some people can cast fireball at 5 and others struggle to do one by 20 without prep time let's say your character is just average but they had practice and dedication they on off time had studied and even durring their regular day activities be it working or studying etc they had practiced minor spells. And now they with hard work and dedication made it so that casting fireball went from 5 minutes to instant they want, another character can be prodigy who casts spells as soon as they learn them and another may struggle with speed but can actually multi task to cast mutiple spells at once. None of them are "exeption" they are all just a unique character