Well, I'd say it depends on the personality of the two cats. I have a 9 year old cat, Shane who has LOTS of problems with my Simba, who just turned 3. Simba is MUCH too playful and agressive for my 9 year old, even though I bought Simba as a kitten, and he's grown up with Shane. Of course, Simba is a Bengal, and he's pretty typical, when it comes to the Bengal nature, however he gets along fine with SunLee, my other Siamese, who is a younger kitty. Simba does not get along well with Angel, who is another Bengal, however Angel gets along with Shane, because they are both more quiet and laid back. Angel is about the same age as Simba. So, I guess age doesn't matter so much, as the personalities of the cats.