Also depends on who is asking. My vet considers all cats and dogs to be senior at 7 years of age. This means that they have to have a full CBC (blood work) before they have anesthesia; even for a dental. And after age 10 he recommends a heart EKG as well!
When I adopted Lacey, I asked him her age. He said 5 to 7 years. I went with the 5 years, KNOWING it would cost me more in vet bills (over the course of her life) if I used 6 or 7.
She had to be spayed and if I used age 7, I would have spent a lot more right off the bat! So now, even though I say she is 7, she could be as much as 9.
Sugar's neck thickened over the winter, the way it does in older dogs. She was 12.5 years when that happened. She doesn't seem to have slowed down at all. In general, bichons calm down a bit around age 5, but that has nothing to do with them being seniors!