We adopted Audrey recently and the shelter told us she was three months... I don't see how that's possible... She's so big. I would guess a year? But, I've never owned a cat, guesses?
It's hard to tell from the pic but she doesn't really look like a 3-month-old kitten, for sure. Does she have any baby teeth left? They lose their baby "fangs" around 4 months old, so if she has her permanent canines, she's definitely older than that.