Wow, 20-25 years? I really did not know that was possible. I have read about someone having one who lived to 18 but thought that was an oddity. I thought 10-12 years if pampered and protected.Yes. But remember, a chicken could live up to 20-25 years. Considering all of the predators and other things that can go wrong, most chickens don't live that long so I don't think I would worry about them not having eggs after 20 years.
A chicken will lay her best within the first two years and then egg production will drop about 10 percent.