To make the meeting between puppies a pleasant one there are a few factors to take into consideration. It's better for your puppy to make friends with puppies his own age because at 9 – 12 months of age he runs the risk of being treated roughly by adult dogs. However, size is not everything in the dog world.
Dogs are pack animals and the leader of the pack is established not only through size and strength, but also as a result of temperament, social experience and the influence of third parties. It's important for dogs to establish a hierarchy, and they get along better once they understand their roles in the pack. The hierarchy consists of the dominant dog, the leader, and the more submissive dogs.
There are signs you can watch for that may help you understand where your dog is likely to be in the dog hierarchy. A dominant puppy's ears, tail and the hair on his neck may stand up when meeting another dog. A more submissive puppy may have his tail between his legs and his ears may be flat. He may also make a low growling noise while in the presence of another dog. It's important to keep the dogs leashed and to separate them if they become too aggressive.
Puppies introduce themselves by sniffing each other. As long as they are leashed and supervised the introduction should go smoothly, and your teenage puppy, aged 9 – 12 months, will enjoy having puppy friends to play with.