I hate to say it, but I suspect Chester is at the beginning of showing his more advanced, wiser years. During the last few weeks, he has retired to our bedroom after I finish the bedtime routine with Moose, around 8:15 or 8:30 PM. Around 9:30 or so, I'll wonder where he is, only to find him sleeping on our bed. He'll look up at me, almost as if he's asking, "Aren't you coming to bed soon?"
Don't let me misrepresent Chester's state. He's his normal exuberant self in the mornings after I finish breakfast ("Let's play fetch!"), after dinner ("Let's play fetch!"), and whenever friends come to visit ("Thanks for visiting! Let's play fetch!"). It's just these little things in the evenings that seem a bit different.
Such a loyal friend. I can't imagine him aging. Then again, he's going to be nine in just a couple of months. Both Wife and I had dogs in our childhood that lived to be nine, so perhaps I'm a bit oversensitive about seeing a behavior change, even as slight as it may be. I'll keep my eye on him a little more this winter to monitor how he's doing. Am sure he's fine.