Knowledge and Power

Magic is based on knowledge- so, for instance, somebody who knew a lot would be more powerful and more in control of magic than somebody who knew less. Type of knowledge doesn't truly matter, but relevant knowledge matters more than irrelevant knowledge- so, for instance, somebody with an encyclopedic knowledge of football would be more powerful than somebody with only a cursory knowledge, but taking a physics class would provide more power to most spells unless they were football related. This is because knowledge connects one more to magic, and obviously some types of magic are more related to certain knowledge. Theoretically, somebody who knew everything would essentially become magic- they know so much that magic is connected to them so much that it feeds them new knowledge as soon as it comes into existence. Connection to magic is also linked to lifespan, since magic can be used to heal and the more you're connected to magic the better it heals and preemptively heals you.

Back before literacy was anything near common but was an expected skill for mages, a mage was expected to take an apprentice but couldn't be bothered to actually teach all the boring-but-essential stuff, so he essentially bought a kid from somebody with too many mouths to feed and not enough money, and did a ritual to give the kid basic knowledge...only he forgot to put a limit on what was to be learned, so the kid learned everything.

All ideas are free for the taking.