Yeah, this currently isn't possible nor do I think will it ever be possible. But if your not too concerned about the NPC itself, you can simulate this with instances.
I've experimented with it pretty recently, I created an instance version of prontera and it's surrounding fields, then I added in all the NPCs. After which, I added in 2 more NPCs that are only apart of the instance. So from this, you could make that instance of the map only available to players level 1-254. Then do some clean up and make them go back to the normal map where the entire server population can co-exist lol.
Otherwise, you better off just making the NPC ignore the unqualified players.