Chickens can live into their teens, although I've heard a few have lived into their 20s. Average is a little harder to say, since it depends so much on how they're managed.
Are yours hybrids, bred for laying? I've heard they sometimes have shorter lifespans like that. Maybe a different breed or a different source for your chickens would live longer for you. If you're breeding your own, maybe try adding some new stock. Make sure the breeding hens are on a good diet, so the chicks develop well in the egg, too.
Do you feed a balanced feed that provides the main nutrients they need? In addition to a commercial feed, I think some non-soy protein and fresh grass, greens, fruits and vegetables help chickens to have longer, healthier lives, too.
There are a lot of phyto-nutrients in various plants that help fight cancer, viruses and other diseases. A great diet boosts the immune system. Typical feed doesn't have all the good fatty acids in it that a more varied diet has. A more varied diet also provides more trace minerals, in addition to the major minerals that feed has in it. I think long term, as the years pass, nutrition makes a bigger difference in the quality of laying and their health.