I come from an ASU school where anyone can wear a hakama whenever they choose to. According to Saotome Sensei (from one of his books), O'Sensei REQUIRED everyone to wear hakama, or they were not allowed to train. Saotome says that they are a traditional part of the art, and that you should wear one as soon as you feel committed enough to buy one - that any association with rank is a new and unfortunate invention, and sexist rules even more unfortunate. His thoughts are the most reasonable I've read on the subject.
Why anyone is in favor of using the hakama for the purpose of highlighting hierarchical or sexual differences between students is beyond me. They are tradition, and they add dignity and beauty to Aikido practice, and that should be that.
On a humorous note, I already had a hakama when I started Aikido (from prior Iaijutsu study). I began wearing it my first week, whereas most people at our dojo wait many months. Further background: I am a fast learner, and am endowed with perhaps too much confidence in many things. Anyway, a couple weeks later, during free practice, I got thrown on my head with a sacrifice throw, because my Nage assumed I had far more experience than I did. Oh well. Also, within the first few months, I got punched in the nose by Sensei, hard! Didn't break it, though. I actually enjoyed the challenge of launching myself into intense practice right away. I got banged up a bit, but it was a great education.