Question: When is female aggression most common?
Answer: Before answering this question, we need to clarify terminology. By "female aggression" you probably mean relational or social aggression. Girls are not, in fact, the only ones who engage in relational aggression. It's true, though, that girls act socially aggressive more often than they act physically aggressive. Because of this, our society tends to associate relational aggression with females.
Relational aggression in both boys and girls is most common during the late elementary to early middle school years. In other words, the tween years are a peak time for the use of socially aggressive tactics, including ostracism and gossip spreading. Even so, social aggression never fully "disappears." It begins as early as the preschool years and continues throughout adulthood.
Leadbeater, Bonnie. Can we see it? Can we stop it? Lessons learned from university-community research collaborations about relational aggression. 2010. School Psychology Review.39, 4: 588-593.