Do I Want an Adult Dog or a Puppy?

Officially seniors by seven years of age, most dogs don't earn the "settled senior" title until they are nine years or older, especially smaller dogs that mature slower than larger breeds.

Cons:

Senior Dog Health Problems
A senior dog may have age-related issues that could be either time-consuming (sight or hearing loss), or expensive.

Ingrained Problem Behaviors
Bad habits that have been picked throughout the dog's life may be harder to train him out of. You can teach an old dog new tricks, but undoing an old behavior is harder.

A Short Life
The biggest reason that senior dogs are hard to adopt out comes from the mentality that people have when they think "why would I give my heart to a dog who is only going to die in a year or two?" Dogs wiggle into your heart rather quickly, and adopting a dog that is ten years old could mean that your relationship is of a short duration.

Pros:

A Full Life
I admit that I have also shared the sentiment, "why adopt a dog just to watch it die"; it's hard to watch a pet you love age and die. But I've also learned that the relationship is a very rewarding one, for both me and my old dog. Even knowing that heartache could be right around the corner, I'll do it again and again after, because the joy is just so much more. And a nine-year-old may still have another four years left to live. Why not help make those years as comfortable and content as possible by giving him a lovely retirement home?

Less Excitability
Senior dogs are wonderful, tempered animals that don't get quite as excited, or quite as anxious as young dogs. They are confident in their skin, and are happy to either walk a sedate mile with you or lounge on the sofa beside you.

Temperament and Training
Most senior have already been housetrained, evaluated for their compatibility with other animals and children, and have had additional basic training.