Buying a puppy vs rescuing a dog. From a dog training/behaviour perspective
When looking to add a new furry member to your family, a lot of people specifically look only for young puppies. There is something to be said about raising a puppy from start to finish. The bond you form is unmistakable. What you end up with when your puppy becomes an adult dog is the direct result of the time, effort and training you have put into them during their formative months. You know what to expect from your dog as you have developed a form of communication together. They have learned to read your body language, your emotions and perhaps even have an understanding of your vocabulary.
The downside of course is the amount of training that puppies require, Puppies have no experience living in a human world at all. Everything is new and exciting. All they know is how to be a dog. And being a dog in a human household means we have to deal with a lot of what we would consider "bad" behaviours. Housetraining can be a frustrating endeavour for most owners. Puppy nipping/biting, jumping up, barking, digging, chewing inappropriate items and a host of other issues can make new puppy parents want to tear their hair out. They made puppies cute for a reason, you know.
On the other hand, rescuing an older dog can be scary too. Most people assume the dog was surrendered due to unmanageable behaviour issues. That the dog is inherently bad. I mean why would the previous owners get rid of the dog if there weren't any issues, right? This fact alone stops a lot of people from even considering adopting an older dog. This isn't necessarily always the case however. Sometimes people are no longer able to care for their dogs due to health, and/or financial reasons. Sometimes dogs are born living on the streets, and have never had the opportunity to live in a home with people.
A lot of rescues will place their dogs in foster homes for a period of time. Those foster families play a critical roll in determining the dog's issues and helping to identify any behavioural concerns the dog may have living in a home environment. They get to know the dog's personality and their likes and dislikes. The foster parents may even start the necessary training and encouragement of appropriate behaviour to ensure the dog's success in their forever home. The foster parents help the rescue to determine the most appropriate home for each individual dog. With this knowledge, you can have an idea of what behaviours you as an adopter can handle and what you can not.
Rescuing an older dog generally means that you don't have to worry about housetraining issues, or the relentless puppy nipping and biting and general destructiveness. Most adult dogs have outgrown this behaviour and have had some previous training.
No matter what age dog you bring home, you will need to train them how to live in your human household. Every home is different, and every human's expectations of our canine companions are different. What is allowed or even encouraged in one home, may be frowned upon in another. Sometimes all a dog needs is a change in environment to help them become the best dog they can be There are no bad dogs, just dogs that need help to become the best canine partner for you.