I haven't processed older roosters so I can't comment on the age thing. But I just wanted to add that you should age your birds in the fridge or a cooler with ice for 24-72 hours until rigor mortis passes. I didn't see you say you did this so that may be why your last bird was rubbery. Once the the joints move freely and you can kind of jiggle the leg or wing easily, you know rigor mortis has passed and the meat is ready to be eaten.