OK, so, let’s say your name’s Sam and you’re a 3rd Kyu Aikidoka. According to the protocol of your dojo, only shodan may wear hakama.
Then, one day, you go to an Aikido course. Aikido from all over the place are attending. At the course you meet Kris. Kris is wearing a hakama. By chance, you happen to learn that Kris is 3rd Kyu, just like you. So, what the heck is Kris doing wearing a hakama?
Not to worry. There isn’t really any established “official” rule for who gets to wear the hakama.
O-Sensei insisted that all students wear a hakama, regardless of the students level. Originally, the keiko-gi was considered underwear. So, to step into the dojo without a hakama was, for O-Sensei, like going to class or work wearing nothing but your grundies. You just don’t do it. So, some dojos allow or even insist that students wear a hakama to class.
Where I train, the hakama is worn once you reach the level of 3rd Kyu. Other schools require the student to have attained 1st Kyu. Still others have students wait till they reach shodan.
Whereas men must often reach a certain level before wearing a hakama, in some places women are allowed to wear one from day one. This custom usually goes back to the idea of underwear and modesty.
So, how do you know when to start wearing a hakama?
The definitive answer is… Ask your sensei.