The basics involve simple protocols in how you refer to each other - you give her a pet name, something appropriate for the animal she is, and use that in private (and in public in kink friendly places). There is a certain degree of roleplaying for her to do - which it sounds like she is already doing - in how she reacts to you and how she behaves. She eats and drinks from a pet dish whenever possible, plays with toys in the same way as a cat would and so on.

Most of my experience is in puppy play and a lot of the elements of that which are really BDSM linked do not apply to cats - things like training (giving her commands and expecting her to follow them as if she were a dog), toilet control (she has to non verbally ask for permission to go to the toilet, using only signs that a dog may use), walking her around on a lead, throwing balls for her to chase and so on. Cats just don't do that sort of thing. You could ask her to wear nothing during sessions other than a collar with a bell on it and, if you really want the extremes, give her a litter tray for going to to the toilet in (though getting her to ask permission to go to the human toilet may be more appropriate for people who don't want what may amount to scat/pee play ).

As for age play, I always understood that to be a situation where the sub dressed as a baby/young person and was treated as such by the Dom/me. Men seem to prefer being a baby (including being forced into nappies and so on) whereas women seem to prefer the 'innocent young girl' thing.

I suppose how deeply you entrench your fantasy into your everyday life depends on how much you want it to be public humiliation for the sub. Is it something that happens only occasionally in play (which is good to start with) when preplanned - say, every thursday between 7pm and 10pm she is your kitten, fridays between 7-10pm she is your little girl - or do you do it every time you are alone in private or is it something you do in 'kink private' (i.e. only with kink friendly witnesses such as at a club or other private party) or is it something you are both prepared to let the whole world know about?