Originally Posted by Mo-Nercy
That is untrue. Although the magic you draw from enemies becomes better, the magic you can refine remains the same. It's actually very simple to end up with 100 Ultimas, Holys, Meteors, Flares, Triples on all your characters, no matter what level they are. As I say, lower level enemies with strong junctions will always be preferrable to high level enemies with strong junctions.
Enemies do level up with you. They're not always at the same level as you though. A Bite Bug can tank two hits from Quistis' Chain Whip at the beginning of the game when she's level 8. At level 100, with the same weapon, it'll still tank two hits.