Chickens can live up to 8 or 9 years or beyond–I just had a wonderful old Buff Orpington die at the age of 13 ½. She hadn’t laid an egg in years, but was a wonderful broody mother to many hatches of my Sicilian Buttercups. Birds at my house seem to live forever and yours will also live longer if you take very good care of them. But the hens don’t lay eggs as bountifully as they will their first and second year.
Depending on the breed you have acquired, the hens will keep laying eggs for some years, though it won’t be as often. Remember when you were thinking about what breed of chicken to get? If eggs are important to you, the egg-laying breeds will lay more eggs for a longer period of time.
Will you butcher the older ones and replace with younger birds? Some people with limited space find having egg layers are important to them and they will do this. You can make some wonderful chicken soup with the older hens!
You can also try to sell or give away the older birds and replace them with pullets every couple of years. You may be able to find folks who are interested in taking on your older hens, though it’s doubtful you will make much money from them.
Will you keep them no matter what? If you don’t care about the egg-laying capacity and just find enjoyment from having some chickens in your backyard, you should have years of fun watching your birds, gathering some eggs and using their droppings in your compost pile.