Meh, they don't have to be 30 but I would be in favour of certain prerequisites being met.

Dark Knight requires a Gladiator or Marauder at level 30.
Gunbreaker requires a Paladin, Warrior or Dark Knight at level 60.

That sort of thing, you'd need at least a general understanding of the role you're going to play before you can dive straight in at level 50+.