It's a common myth that older shelter dogs are damaged goods with behavior issues.
Many animals in shelters, especially adults, were once members of loving families, but for various reasons, ranging from allergies to cross country moves, were given up for adoption. Many of these pets are house-broken and have had some obedience training and are just waiting for their new forever home.
Another good reason to adopt an adult is they are a little more relaxed and easier to handle. As animals mature, they calm down and become more confident while still being very playful.
You will also get a better night of sleep. If you have ever raised a puppy, you know they will cry all night for attention while adults will likely sleep all night and much of the day if you are not home.
Small dogs, on average, live longer than large breed dogs and you may want to factor that in when making your decision, but most dogs live into their early teens regardless of size or breed.
When you adopt a senior dog you are not only adding to your family but saving a life. Senior dogs are very often passed over and many will never find a forever homes.