If you are Alliance the first "real dungeon" I believe you can enter is SFK at lvl 10. I always go to a dungeon as soon as I can get in. My thought is that even though quests can give great experience, with a booster in a dungeon there is no downtime for turning in quests etc, just raw experience all the way to 61. If you have other RAF toons, I would set either a DK or one of the RAF account toons at the enterance of each dungeon, this way you can summon to each instance and have no travel time for the lowbies.
Just my 2 cents