I have had some that lived to be a little older 4 years old.
The girls were dust bathing on this day.
My girls were too big to fly on the roost. I sometimes would put them on the roosts, if at coop closing time, their eyes were sparkling with envy to get on the roost. I would lift them up and place them on the roost. I had to take them off and place them on the ground the next morning. Sometimes, they just chose to sleep on the ground in their chicken house. I made their ground sleeping arrangements as comfy as possible.
I took my time and studied the breed to find out how I could keep them around longer. Of course there was very little information to no information out there because no one really keeps this breed around a long time. There are a few other people on this forum who have raised Cornish as pets and I am one of them. My girls had an excellent chicken life. Anyone who knows me know that my animals have 5 star status in terms of health, nutrition and living arrangements. I wouldn't have it any other way.
This is what I did that allowed them to live so long.
1. Do not overfeed.
2. Free range as often as possible (daily if you can). Free ranging also promotes exercise. They have to walk and run to get those juicy bugs.
3. Place feeder several feet away from them, so that they have to walk (exercise) to get their food.
4. Feed healthy snacks (fruits