How old was Geraint when he learned he had powers? How did he react? And when he first started using illusions did they effect him, like whatever he tried to show other people did he see it all as well?
How did everyone else with powers react when they learned they had powers and how old were they? (Galahad, Percival, Bors, Lancelot and Arthur, etc.)
It's slow moments at work so I guess it's Lore Time™.
So I'm still figuring out the intricacies of the magic system of the knightverse, but what's important to note is that everyone has some degree of magic in them. The magic tends to manifest in three ways:
1) Divine/Otherworldly. This encompasses the gods, minor deities, Fae and Fae-born, etc. Their magic is on another level that even the most skilled and learnèd of wizards can't hope to achieve.
2) Wizards/magic users. This encompasses people like Merlina, Ælle and Ban, and also people like Yseult. For the former case, there's a magical affinity in which learning new forms of magic is considerably easier than others and their abilities to master them exceeds most others. In Yseult's case, she has a particular affinity for healing (or Life Magic) and all the branches of magic related to it, but it's not a passive ability. These sorts of people never cast their magic by accident; it must always be intentional.
3) The Rest. Practically everyone else falls under this category. It's hard to discern the line between less powerful magic users and very powerful people of this category, but a lot of it has to do with when the powers manifested and whether or not they can be used passively. The Rest (for lack of a better term) tend to gain their abilities from a very young age and can use them without thinking (though practice still makes perfect!), but they will never be able to gain branching abilities; what they get is what they get. More powerful people in this category are Lancelot, Gawain, Geraint, Lamorak, Percival and Galahad. Less powerful people include Uther and Igraine (Uther's magic manifested in a Jack of All Trades type of skill gain, Igraine's in a knack for learning and remembering healing methods... "Talents" if you will).
So, to answer the actual question, Geraint (as one of the Rest) got his powers young and he was very difficult to raise as a result. Geraint sees the illusions he makes, but he's long since learned to never rely on his eyes.