If you continuously breed them and expect them to lay eggs, increasing lighting, etc...their lifespan will diminish as their poor bodies go through so much. You can get great fertility from 2-3 year olds if you give the birds "breaks" once in a while..decreasing their lighting, letting them rest, regain their strength and energy. I have a rest pen that I put hens in the winter time. I rotate birds around as well. While the ones I put in the rest pen, the newer layers go into production.
Again, if you continuously breed the birds, their fertility does drop with a year, as they are physically exhausted. Some people add electrolytes in their water, which quickens their lifespan further as they are popping out more eggs than their bodies are naturally made to do..