Yes...In fact, many people stay away from tough quests, or quests they just don't like, until they get to higher levels. You won't get XP for quests that are too low of a lv but @20 you will still get xp for lv 18 quests with no xp penalty (starts at 2 lvs above the current quest (at -10% up to 0% at 7lvs above)). This depends on what difficulty you enter the quest on (ie. ML 18 on normal is min lv 20 on elite). Regardless of the lv difference, you will get full favor rewarded for the lv you complete it on.
One thing to remember (unless it has been fixed), if you run a heroic/epic quest for the first time on Epic it will only count your highest epic lv completion for favor. So, if you run a quest on EN and then again on HE you only get favor for Normal completion.
This page explains it further...
Experience point 2.1.12. Just scroll down to part 2.1.12.
You just have to make sure you select the Heroic option instead of the Epic (which will be the default) on the quest entry window.