as far as I know it varies between dojos. Some dojo starts from 6th kyu, some on 7th or maybe 10th as well, depending on the organization they affiliated with. This also the same to the testing requirements, but usually they looked to your number of hours of training, the material, though can be varied according to what has been set by the organization.
If I was attacked by thirty ninjas riding T-Rex's, supported by a vast undefeated clone army of robotic Lincolns wielding Febreeze, should I Tenkan?
Tenkan to your Terminators, let them do the rest. Nice saying,though.