They're locked in at the level you picked up the quest. The quests actively scale with you, when they havent been picked up. I.E. "Bad Hair Day" is available for pick up at 51 for as long as you are 51, and as soon as you level so does the quest. So just cap, then run your unique quest reward granting missions :P
Edit: that is kind of convoluted.. Sorry I'm not able to be clearer Make sense?
Assistant Manager at Foveroula and Acid's Pawn Shop I remember Alvor every time I make Skyforge Steel