I have two adult cats, Mika and Kimi. They're approximately 9 and 10 years old, respectively. We also just added a kitten to the family, yesterday in fact. We're not sure of her exact age, somewhere in the neighborhood of 8-10 weeks old. Things are going fairly well as far as the introduction process, Kimi got a chance to sniff around where the kitten had been last night and Mika seems mostly apathetic about the mysterious noises that they suddenly started hearing.
The kitten (tentatively named Stormy) is a bit skittish and isn't litter trained, so we're working on that, but I'm sure she'll settle in nicely with time.