Lets see. 5 months old. If she just got that comb in the last month that means she is laying or ready to lay. If I see those tail feathers right, and she has no hackles or sickles... It's a girl. Mature girl. Congrats if you wanted a girl and sorry if you wanted a boy. It is 100% pullet.
definitely a hen. Remember that Black Australorps are known for their huge combs!! Haha, thats not even half the size of the comb my BA hen had though. Try not to judge just by comb, that normally doesnt work, everyone always thinks its a roo if it has any type of comb.