Many years ago, I was told that our season opener of april 15 is based on the idea that by then 60% of hens are bred (in a normal year). If you live here, its hard to miss the obvious age variety of poults. By late summer, there is evidence of a second hatch which is most likely because the first hatch goes bad because of weather. Im uncertain as to whether or not first year hens cycle later (if at all) than matures to present this difference, but it could be.