Aging dog, aging dog

Lacy, a 10 year old fluffy white, fearful dog is rapidly decreasing in spunkyness. She used to climb mountains with us and run next to us as we biked. But now 30 minutes of walking tires her out very much. Once she was a spunky pup, but now just a fat old sausage that yearns for her family. Incredibly lazy she is when she has been away from her husband and son for a long time, but when they meet she perks up so much that it's amazing! But just weeks after thay are separated she turns back into a lazy old lady, not that she is that active when they are there;just a lot more than without.
But not all dogs get that inferior as they age, for instance Cheops, a spunky 10 year old schipperke with flat black hair. Sure he used to run a bit faster and wanted to play a tincy bit more often, but he hasn't changed that much. That is, excluding look, around age 7 white hairs began to fluorish into his fur and they are still coming, yet these mainly come from a dog being stressed. It just happens to be that the older they get, the more stressed they get, and when one gets stressed their hair dies (some say soaking the white hairs in green juice restores it back to regular health and color). Yet that isn't the point, when your dog ages it may make a huge difference or hardly change a thing.