Quote Originally Posted by Flying Mullet
Does this mean that you only have a certain "level window" to learn the abilities in? Granted, if you're level 40 a monster with a level 5 ability wouldn't be very useful, but if someone's a completist nut then they're out of luck it seems like.
Yes and this is only an issue with a very very very small number of monsters. Considering how slow levels are gained in FFXI if you pay attention you should not miss any.

Among the very very few in the game you'd have to have missed something like Sheep Song at level 16 and your last chance to get it is level 50ish. Unless you are totally oblivious you can't really miss them. Additionally I have the feeling SE will put higher level versions of these mobs in some of the new areas to curb this potential issue.