Most of the base of my laying flock are 5 and 6 years old, so if I find one dead I wouldn't be a bit surprised. But...the last bird that just up and died in my coop was a Black Star that had made my cut into the 2nd year....barely. That was 3-4 years ago.
Sometimes birds just up and die. It happens. I've never had one die outside of just natural causes, though....I don't know exactly why that is. All the ones that have died just dropped off the roost and I found them the next morning. No previous symptoms of acting ill, not eating or not laying....just fell over and croaked.
That's life with chickens, particularly if you don't cull the whole flock every 2 years.