Here's the thing. They don't usually slow down 'til they're dead. That's the magic of a dog. They love to play at any age, they're ready to do whatever you want to do whenever you want to do it.
For the life of me, I can't imagine anything other than full-speed all the time. That's what makes a dog a dog.
I had the pleasure of watching 8-year old Tango chase 6-month old Ike around and around the house today. It's too cold to stay outside, so the dogs do their playing indoors. Sure, the house gets a little messy, but a little mess is worth all the smiles that result from watching an adult dog play with the same enthusiasm as the youngster.