I sell (or eat) birds at 2 years of age, but the one I did end up keeping the longest was still kicking at almost 9 when she was given back to my neighbour. She might be still living, who knows. I'd say 6 is a good average age for a bird.
As of now the oldest birds I have are 3 years old and still acting like younguns.
I have very few birds die from anything other than predators or accidents. Let me see what I can remember...
EE cross hen that was probably about 4 turned up dead one day. She was a really weird bird, chronically skinny and she sounded weird. Should have been culled a long time before she died.
Buff orp hen that turned up dead at 3--4 for no known reason. Wait, she might have had blood on her? I don't remember, that was 4 years ago.
PR hen dead in the nest boxes one day. Maybe egg binding, I didn't do a necropsy.
Leghorn hen that was prone on the floor after hearing thrashing in the coop. Maybe she got knocked off the roost---I don't know. She died that night.
PC chick that died at 1 month of age, cause unknown.
2 cockerels that died from weak constitutions combined with negative degree Fahrenheit highs for over a week.
RSL hen or two to EYP, age one
PC pullet to Marek's
EE hen to eye infection, age three
OEGB hen found dead in the feed box. Probably about 4 years old but possibly much older.
Probably a few other layer birds I've forgotten
I think that's about it, really. I've had close to 200 fowl, counting some birds sold at a few months of age.