Cole is 11 mos and has been on adult food since about 6 months.
We use Blue Buffalo as well and he was fine with the transition.
Our vet recommended it b/c Cole was growing very quickly and on the puppy food he was getting very chubby! We switched over gradually of course, over a 2 week period, but it was just fine!
I dont remember when we switched Roscoe over but I think it was pretty early on too!