As all true pet lovers know, senior pets are special. They have the wisdom of their years, having been around the block (literally, in many cases) more than a few times — yet they can still be beguiling, funny, and endearing.

Senior pets are also especially vulnerable. Millions of senior dogs and cats live in shelters around the U.S., surrendered by owners who can no longer care for them or simply don’t want to have them any longer, sadly.

Millions of senior dogs and cats are euthanized each year when time and shelter space runs out.

“The saddest and most maddening reason for surrender we hear is, ‘The kids outgrew him,'” a shelter volunteer on the North Shore of Massachusetts told LifeZette recently. “We think, ‘What’s up with you? Can’t you take care of him?’ They didn’t realize that the cute puppy or kitten was a lifetime commitment.”

Senior pets often have lived with a single family for their entire lives — which makes their transition to a shelter especially hard.

The top reasons for surrender tend to be more serious, such as relocation or death, financial difficulties, or lifestyle changes such as divorce or job loss, according to Petful.com.

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But many senior pets still have a lot of love and companionship to give to a family. While their time may be limited, senior pets more than make up for that with their loyalty, their sense of calm, and the knowledge and training they already have under their collars.

And sometimes, magic happens between a senior pet and a human being. “I walked out of the shelter in a daze, holding this still-shaking senior Chihuahua,” Maggie Clancy of Hollywood, California, wrote in an article for Dogtime.com not long ago about her sudden adoption of an aged little toy-breed dog.

“I had been listening to a lot of David Bowie on the way to the shelter and found myself singing ‘Jean Genie’ to her. ‘That’s your name now,’ I said. ‘Genie.'”

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Genie was a perfect match for Maggie.

“Almost two months later, I cannot imagine my life without Genie,” she wrote. “I don’t want to think about what would have happened to Genie had I not been possessed by some force to go get her.”

A whole market has popped up to support owners with senior pets — everything from supplements to mobility aids, wheelchairs, and booties.

Purina Pro Plan has entered the senior dog market, developing a senior dog food called Purina Pro Plan BRIGHT MIND 7+, which contains enhanced botanical oils that the company says promotes alertness and mental sharpness in dogs aged seven and older.

“Dogs are just like us; they need a good diet, lots of exercise and mental stimulation,” Dr. Brian Hare, one of the world’s leading canine cognition experts, noted in materials provided to LifeZette by Purina Pro Plan. “It sounds simple, but doing these three things can really help your dog reach his full potential. Nutrition in particular becomes especially important around age 7 when the glucose metabolism in his brain begins to change.”

If you do take a chance on a senior pet, you will change lives forever — yours and your new pet’s.

The unforgettable relationship will be marked not by its length, perhaps, but by a bond forged by a dog’s gratefulness and a human’s compassion.

“He ran around our backyard. He cuddled with our other dog. He wagged his tail, he loved to be petted, he loved to go on walks — he was truly a gift in every way.”

One Massachusetts man recently had to euthanize his senior mini-poodle, whom he and his family took in unexpectedly when she was seven.

“Baby was one of the sweetest dogs I’ve ever come across,” he said. “She was so thankful and happy to have a home where she was loved and cared for. She gave back more love than she got. I wouldn’t have traded being her ‘dad’ for the world.”

And a New York woman will never regret taking in a shelter dog in his later years — even though, alas, he only spent three years with her and her family as his time came to an end. “He joined our big family and I can’t even begin to describe the joy we felt in seeing him happy in his last years on this earth,” she said. “He ran around our backyard. He cuddled with our other dog. He wagged his tail, he loved to be petted, he loved to go on walks — he was truly a gift in every way.”

She added, “It makes me incredibly happy knowing we gave him a chance, no matter what else he’d experienced earlier in his life. And I think our kids learned the lesson of giving in a very profound way.”

Here are some facts about senior dogs, as noted by CesarsWay.com:

  • Senior dogs at shelters need homes as badly as younger dogs. Many older dogs were once owned and loved by someone.
  • Older dogs are not always “problem dogs” (though some, of course, will have physical issues in their later years). Senior dogs lose their homes for many reasons — often because their owners are unable to keep them.
  • Older dogs usually come trained and know basic commands. Most older dogs have mastered such commands such as “sit,” “stay,” and “down.”
  • Dogs can be trained at any age; older dogs are just as smart as younger ones. Older dogs also have a greater attention span than puppies.
  • Older dogs are calmer and less energetic than younger dogs. An adult dog has an established demeanor and temperament.
  • Older dogs make instant companions. Unlike a puppy that needs training, an older dog is almost always ready for a walk.

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Deirdre Reilly is a writer and editor in the Boston area.