Or it is worn by males and females 6th kyu or higher (ASU), or males and females 3rd kyu and higher (I think) (SBK), or males and females, Dan rank only (I think) (Iwama), etc, etc. One Nihon Goshin dojo I attended, even the instructor (a shodan or nidan, as I recall) did not wear one. The only thing with more variation is 'the right way' to do ikkyo.
I don't really care. I didn't have trouble learning to move in mine, but my first dojo made you keep your feet on the mat at all times, so I think that helped. I also learned to fold it there (since we could fold our sensei's) so again no real trauma in transitioning to owning my own. I don't see that it makes me more graceful (
) or more upright (
) as some have said it does for them. I think it is a bit warmer in winter, but also in summer, and since I don't wear one in my current place, I miss it only sometimes when I go to sit down and catch myself 'moving' a phantom hakama out of the way.
I don't know if there is any time to say it is worth the investment, as folks quit Aikido at all stages, sometimes after shodan. Besides, having just decided to take up golf, a hakama is actually a very cheap sports investment. As for hiding the feet (and hence possible mistakes) in students....well, I have spent 3 years now watching my instructors feet, and it is possible to do, even in hakama...but if a problem, beginners could just hike their hakamas up for a footwork inspection...
It seems to me, that wearing the hakama from the earliest stage in Aikido would be the most traditional, in keeping with O Sensei's teachings, thing that could be done. I just don't see that we always need to do something just because that's what has always been.