The majority of my chickens live to be 10-12 years old and they are a good cross section of breeds. The aging process is very gradual. The EE I had that lived to be 18 started sprouting white feathers all around her head. The eggs that she was laying were almost flat (think of a "deflated" egg). She had arthritis in her legs but that started when she was about 12 and progressed very slowly. Their feather quality declines , again, nothing you would see overnight. Usually an old chicken will succumb during a time of stress such as going through a molt, extreme cold or heat.. I used to bring my EE in the house during those times and I saved her from certain death a number of times. Their immune systems are really weak so they are really susceptible to mites. Almost every old chicken will develop a bad case of scaly mites unless you treat it aggressively. Towards the end, the other chickens may begin picking on and chasing an old chicken, usually into a corner where he or she eventually die if you don't intervene. Something that they generally don't do to chickens that are just plain ill. I think it's their way of hurrying the process along.