When is playing not playing anymore?

I've had Boogs since September and Gracie since October, they are about 4 and 5 months old (ish). Anyway, they have been best of pals, bonded instantly when Gracie joined us. They do everything together and usually Boogs does whatever Gracie does. They play together, sleep together, and groom each other.

Lately, they have been playing quite rough and usually Boogs is dominant in play - she has been chasing Gracie a lot. Also, lately, the play turns into what seems as if they aren't 'playing' anymore - hissing, growling etc.

Does this have anything to do with their ages, and do cats have a pecking order like dogs? Maybe that's what they are trying to sort out.

Any ideas?

When my Indika was out of sorts, she would chase Neko around the yard climbing on her like she was a male. I was told that since there were no male cats in the whole neighborhood, she was asserting her dominance, so yes I do think they do it to establish a pecking order of sorts.