I have a 2 year minimum rule for myself. A few months ago, the ages went 11.5, 9, 7, 5, 2. If I still had those same numbers and you added in my newest dog, she's 1.5. Kind of throws off that nice little list above, but in my defense the youngest had just turned 2 before the new one was added.
I just feel like it is easier to bring in a new dog (regardless of age) once the current dogs are trained, under control, and know the rules. I couldn't imagine getting through house-training, chewing, basic manners/training, etc and then turning around and doing it all over again a few months later. But that's just me.
As I said before, I fostered a lot for BCR. This allowed me to see what their temperament, personality, etc was like before I committed. I wanted to know 150% what I was getting into. First and foremost the dog had to get along with all members of the pack, human/canine/feline. Second, I had to be able to tolerate the dog. As many people know, sometimes your personality and the dogs personality just don't mesh.