YumYum was about 4 or 5 months old when we got Dutchess who is 5 years old. We kept them separated for about a week and a half and would only let them out when we were at home. They are now best friends and will actually sit next to each other. They will even sit on daddy's lap together.

http://pepeirce.tripod.com/thekittycorner

Good luck with kitties!