The Nar Shadaa bonus series starts at level 28. The quests are level 31.
Some planets require you to be a higher level to start the bonus series. Alderaan's bonus series is level 40, for example.
Speaking of, is there any breadcrumb at all to the Alderaan bonus series, or are we just supposed to randomly go back to Alderaan at level 40 and find it? I had no idea there was one until a guildmate mentioned it (and I was already well over level 40).