So the chicken was created mature and fully functional.
"First of all, God obviously created things that were fully functional from the beginning. After all, plants had to be bearing fruit in order to provide sustenance for Adam, Eve, and the animals. He did not just create seeds and wait for them to grow. The created “kinds” had to be capable of reproduction, so they were not created immature. As mentioned, Adam and Eve would have to be able to reproduce in order to fulfill the mandate to be fruitful. This does not imply that these creatures were “created old.” It merely indicates that they were created functional."
-by Dr. Tommy Mitchell on October 26, 2010 / Answers in Genesis
