Story quests are like this actually - the quest is often available to start one to two levels lower than the actual level requirement, but cannot be completed until you have actually reached that level (the quest will have a red 'locked out' symbol over the NPC even if the Duty List shows objectives completed and that you can turn it in.). It's caught me off guard quite a few times during the storyline, to the point now I take note of the actual level requirement for the quest and if it's higher than my current level I hold off accepting the quest until I reach that level.