I thought Chiba sensei wasn't allowed to train in the whole of his first year? So it might be 11 months after he was first allowed to train.
The hakama is basically just trousers anyway while your gi trousers are underwear :P Osensei apparently used to tell his students to wear hakama right from the start, even banning some people to train when someone who owns one forgot to bring it. He said that it's rude to train in your underwear
: But then lots of people still trained in gi since they couldn't afford a hakama anyway. Oh and in rendez-vous with adventure we can see the host of the documentary wearing one right from the start even though he just started.
I think in hombu and lots of dojos the tradition is that women can wear a hakama from 3rd kyu while men from 1st kyu. I've heard teachers letting students wear hakamas early due to special reasons or because they started teaching early. I guess it's just politeness for women and for respect of people who assist in teaching.
As with what people wear underneath, I've heard of someone who made a hole in his gi pants to make going to the toilet easier... It's always quite funny when people in hakama really need to go