Also, puppies should have at least one set of shots before going to homes. As for the other end of the age spectrum, older animals. I am a firm believer that any dog at any age even a senior will have something to offer. If you go to a rescue, look at an adult dog. Just because a dog is fully-grown does not mean they are past training.
Adult dogs have better bladder control and more attention span than a young puppy.
Sometimes puppies in rescue may have to be placed younger than eight weeks. This is an exception to the rule and many shelters will try to find foster care for young puppies if possible. The younger you get a puppy, the more work it will be and the more patience you must have with it. Ideally, no pup should be placed before eight weeks. If a breeder tried to insist otherwise, get out.