One of my Australorps (Gigi, we call her) hatched a baby in the dead of winter. She was also sitting on our Americauna's eggs, but they didn't hatch. They developed and one even pipped, but they died shortly afterwards or something. So she only had one baby. The Americauna eggs were from pullets that weren't a year old, so I'm thinking that's the reason for a failed hatch. My Australorp hens are all 8 years old, so I'm surprised she got one of her own eggs to hatch!
Not to bring you down or anything, but all books that I've read say that chickens have the best hatchability rate right around 18 months to two years of age and for awhile thereafter. But don't quote me on that, I'm going by memory and I'm not the sharpest person... ~ Oaknim