(Photo courtesy Schiffer Publishing)

Nancy LeVine's book includes pictures of dogs as old as 21.

It may be true that you can’t teach an old dog new tricks. But humans may be able to learn a lot from senior dogs, including how to deal with aging.

In her book "Senior Dogs Across America," photographer Nancy LeVine includes pictures of pooches as old as 21— or 147 in dog years— from coast to coast. Some of the dogs are still working on ranches or farms, while others are just curled up on the couch.

"There's this sort of palpable love that moves back and forth and such an implicit trust," LeVine said about what it's like to age along with your 'best friend.'