Hi Daisy, are you talking hybrids or pure breeds?
Hybrids were developed for the commercial farms and as they are usually spent at 18 months, they only have a life expectancy of 2 years, but with a good home they can last 4-5 years. However because they only have a certain number of eggs inside them which are laid nearly every day for the first 18 month-2 years, they soon run out.
Pure breeds however can live a lot longer 12 years isn't unusual and because they don't lay very much (if at all) in the winter, their egg laying years are longer. someone said the other day that chickens can live for 30 years, but I've never heard of one lasting that long.