Magic’s anti-aging properties don’t work like this, though. It keeps you at a young age for longer, not an old age for longer. And gaining magic doesn’t immediately slow your aging, the more you train and use your magic the longer your lifespan gets. The more powerful the mage, the slower the age. A very old wizard who’s only a few years away from death like Enzior wouldn’t be able to train his magic enough to slow his death by any significant amount, since he has already finished almost all of his aging.
I’m guessing Enzior has been alive for a few hundred years already and is now at the limit of how long his magic can keep him kicking for.