Is there a level limit on when you can join the Dawnguard? Can I join from level 5 for instance?

The only thing I've noticed for both Dawnguard and Dragonborn is that both quests seem to occur after I've met with the greybeards for the first time, not sure if it's because of this quest being a gateway for the DLC's or if it's because I hit a certain level at this quest or not.