Both the cats in question are neutered males. It does seem something triggered the aggression all of a sudden, as the older one treated the younger one as a buddy when he was 2.4 kitten when we brought him home in March of 2010. We give them a small amount of fancy feast every morning and evening as a treat (thus their learning when I say snack time). They still always eat it together. The younger used to jump over the older one's butt since he was small. He doesn't do that now, and is keeping a wary eye on him. We are hoping it will sort itself out, without having a keep them separate. We are very careful to make sure all the cats get attention.