I have a 2 year old female cat who is used to other cats that we have in the house.,
Next, i adopted a 1 year old female, who came into the house with no problems at all. No hissing, nothing.
Next we got a 6 month old male kitten. Fur flew everywhere! My 2 year old hated him!
Now we introduced a brother and sister who are 1 year old into the house. Well, she only hates the male again.