If you are talking about Cornish Rock/Cornish X chickens... well, they're not really meant to live long at all. They're bred to grow very fast and be eaten. I have heard of people keeping them... but they tend to have a lot of health problems and often drop dead or break their own legs, especially if they don't have restricted feed and aren't forced to move around and exercise. Honestly, if they are the Cornish X, I am pretty surprised they've lasted this long.
Once ours started panting 24/7 and practically refusing to walk (only getting up to go to the food dish to eat) at... 10 weeks I think?... we knew they had to go, despite them being my babies, my first chickens, and having names- Ivy and Iris.
By "had to go," I mean, we found a local place that processes chickens humanely for people to eat, and took them there in the morning. They killed them humanely (that is why we chose the place) and we picked up their bodies in the afternoon, looking just like the chicken you get from the grocery store, just HUGE. ate one already, have one in the freezer. If you are attached like I was, it is hard, but we knew it had to be done because we didn't want them to suffer.
If they're not deteriorating like most Cornish X chickens do, they may have a respiratory disease. But it's probably because of their breed/age/size/overall health from growing so quickly like they're made to, so it's most likely that.