Age of an a/c is not true indicator of its use. Number of cycles is the most logical measuring stick for airlines do decide whether the a/c will stay or go. Other considerations, such as age (obsolete technology), fleet commonality, availability of spare parts, noise restrictions, etc... play parts in the decision as well.
But generally speaking, old airplanes are the first to go, rather than new ones. Then again, DC3s out there defy that logic.