In my dojo we are allowed and encouraged to start wearing one after we pass our first (6th kyu) test. I bought my first one a few months after I passed but had an old one on loan from the dojo until I was able to buy my own.
I really like wearing mine. In fact the few tomes Ive trained without one on since starting. I've kinda felt naked. LOL sort of like I was training in my underwear.
Best thing to do is ask your teachers what the rules of the dojo are on wearing hackama. As has been mentioned every place has their own way of doing it.