The Bannies must be some kind of intelligent medium sized dogs, like standard poodles. They start dying of old age at age 10. However, they also cease procreating fairly early, even given ideal conditions of single family dwellings.
I currently have a town that is going to die because they stopped reproducing and no nomads have shown up. And yes, I have the necessary conditions for nomads, but not until the town was fairly old (about 10 Bannie-years).
My population just fell from 30 to 29, and there are no children, nor students. Unless I get some (young) nomads in the next year or so, I expect this town to die of old age.