Average age of the flock is 3, due to large influx of youngsters last year. Average age last year was 3 as well, but I didn't do fall culling. I have some birds that are five and still giving ~4 eggs a week in summer. (Nothing in winter, though.)
The average age of chickens has a lot to do with a lot of things. In theory, chickens can live 8 to 10 years. In reality, few make it that far.
--Some people (and virtually all commercial enterprises) butcher chickens after 2 years when their productivity drops.
--Predators kill many chickens before their natural life is up.
--Illness and disease will take others. Chickens, particularly hatchery stock, are very prone to reproductive disorders. I read somewhere that fully 1/2 of hatchery stock layers will die of reproductive disorders by the age of 5.
Putting all that together, I don't think the average age of a non-commercial, backyard bird is more than 3 or 4 years old.
Mathematically, we were talking about this in school this week, if you did it on a line graph you would see high numbers of 0-2 year old birds, as the birds get older, there are fewer and fewer. I have seen many posts of birds over 6, however, I myself have never come close due to predation mostly. Age caused illnesses rise with the age of the bird, they tend to die of natural causes with age, and people do cull them. So much fewer older birds, much high number of younger birds, I am picking somewhere between 2-3 is an average age.
If you are hoping for a long term investment in birds, think of investing or keeping of a flock for years, the birds in the flock flow in and out of it.