I am interested in other people's answer to this question too. My game plan is to start replacing when i see a drastic decline in egg production. I'm not sure at what age to expect that though.
Due to the problem with "chicken math" We keep the old girls and just get or hatch new chicks every year. So, in the end, every year there will be more birds and coop size will never be large enough.
Love it!
I know, not very practical but I do not want to eat a tough old bird anyway.
Typically, they will have their first heavy molt around 18 months, though this does vary, and will lay fewer (but larger) eggs after that. Some will lay some eggs for several years or even all their life. I have 6 that are over 3 years and get 3 or 4 eggs a day; there is one younger one who I'm not sure is laying. The blue eggs used to be medium and are now at least an XL. I have some chicks coming along but don't plan to replace anyone.
When I was into this phase I'd sell my layers during their 3rd year--I'd get them in the spring, they start laying that fall then I'd sell them during their 3rd summer. I found there was a better market for them at this stage since they were still fairly reliable layers. I no longer do this so am running a home for aged chickens since I'm not interested in production any more--just need a few eggs.
I just got my girls in Sept 2010 so hopefully it will be awhile before they need to be replaced. But when that day comes I will just have to build a retirement coop it just does not seem right to cull them after they provided you with years of fresh eggs. And not to mention they are really friendly and will let me pick them up and hold them like my cats and dogs will.