Our cat Casey is 3 years old, and our dog Banjo is one and a half. We also have a 6 month baby in the mix, so our house can be a little crazy at times!
I will admit, Casey wasn't happy initially when we brought Banjo home. I think he was disappointed that his place as our only fur child had disappeared. However once he realized that Banjo was here to stay, he warmed up to him a lot. I would honestly say these days they are best friends - the odd duo, but they play with each other, sleep next to each other - it's very sweet.
They have actually been great with our baby too. Very caring, loving and protective. If she cries, they come on over to see if she's doing okay. I think since both pets were here before the baby arrived it worked out well, because I don't think they felt jealous or threatened - if anything they were happy to have a new playmate!