I had three dogs, adopted at different times, but it ended up that they were all about the same age. I started last year with three dogs and ended up with only 1. It killed me to put them down, but it was best for them. When the time comes, what *I* want is immaterial ... it's what's in *their* best interests.
The last of the 3 is Katie: her entire litter was tossed out the driver's side window of a car into on-coming, rush-hour traffic. She was the only survivor, and lost her right front leg to radial nerve damage. She was only 5 weeks old. Katie is now 15 years old, and in the early stages of renal failure. And she's incontinent. And used to being a spoiled, pampered Princess. Her remaining time with us, however, is limited - I know it in my head even if it don't want to acknowledge it in my heart.
In December, we adopted Puddin' ... our little Yorkie. It's the first time we've ever had a dog this small, and it's what we really need at this stage of our lives. Puddin's life span and ours should be about the same ...
My hubby has recently become disabled. He was sinking into a depression that was not nice to watch. Puddin' has given him a whole new outlook on life, and the change is remarkable.
Well, now that ya'll have TMI (Too Much Information

), what I'm trying to say is this: it hurts to have a beloved companion age on you, and it's one of the worst things that can happen to you when you have to take them to the vet for the final time. But when it's compared to the lifetime of love, devotion, companionship, entertainment ... the whole gamut of emotions ... it's worth it. And having another beloved companion at home to comfort you when you get home is beyond compare. Nothing soothes an aching heart like puppy kisses.
There is a cost involved in maintaining elderly dogs, but there is ALSO a cost involved in not having a younger dog around. Besides, I think a puppy helps keep the older dogs more active, gives them a little physical and mental stimulation that we, as humans, just can't provide for them.
It's worth whole-hearted consideration.