Age difference in dog "siblings".
I remember when my son was a toddler, there was much discussion among my friends about spacing of siblings. Some of my friends wanted to have their children close together in order to get the "bad parts" over with. Others wanted to take time to get their lives somewhat back in order before delving back into diapers and sleepless nights. Also discussed was what was best for the children, would they be star students and subsequently successful adults depending on how much one and one time they had with mom or dad and how the other children in the family affected the outcome.
Lately it has come to my attention that this dilemma faces dog owners as well. While I'm not sure they discuss this at their weekly playdates at the park, I know that most dog lovers dread the day their dog will be gone and frequently plan ahead by getting a second dog in advance of this inevitable event.
We have had a number of consults lately with people who have dogs who are just hitting the age of maturity and are "suddenly" developing aggressive tendencies. Upon gathering background info we find that the dog also lives with a geriatric dog (older than 10 or thereabouts). Most of the time the 2 dogs seem to get along famously until they don't. The owners don't generally recognize an event to change the status quo, but there usually is something. What we have found though is that people expect the older dog to "raise" the younger dog in many respects, but this is not usually an older dog's dream assignment. Like you, they may be relishing their retirement and are then handed a biting, spastic whirlwind of a puppy and are expected to play nice and tolerate the abuse a puppy can dish out. To make matters even worse they are frequently punished when they have to resort to correcting the puppy because the humans in the house are oblivious or maybe just too busy. This in turn doesn't help the puppy because they miss out on proper social queues and etiquette because their main canine role model is just trying to stay away and find a nice place to nap. The older dog resorts to growling and snapping every time the young dog approaches and the younger dog eventually learns it's better to just avoid the older dog. Eventually the younger dog matures and at some point will determine it's time to move up the rung by knocking the older dog down a rung or two. This can be precipitated by a something as obvious as a move to a new house(new territory, new order) or as small as a scrap of food hitting the floor and a battle ensues(MINE!).
This doesn't mean that you shouldn't get a puppy when you have an older dog, just that you need to be ready to address the specific needs of both dogs in order to make it a successful transition.
1. Make sure the older dog is in good health overall, nothing makes a dog crankier than not feeling well and then being expected to play nice.
2. Train the puppy to respect the older dog. You must step in when the puppy is playing too roughly, just as you wouldn't allow a toddler to scratch and bite your teenager. You should be the disciplinarian, not the older dog. The older dog will usually give a verbal warning when the puppy is overstepping, this is your signal to step in and re-direct the puppy. When very young, just remove the puppy and give him a toy he enjoys. When he gets a bit older and has learned some basic obedience, call him away from the older dog and have him do a sit or a down in order to wind down a bit.
3. Train the older dog not to bully the puppy. The best way to do this is to make sure he doesn't have to defend himself! If he has to snap at the puppy, you've let it go to far. On the same note, if play gets out of hand and you miss the queue, the older dog does not have the right to hurt the puppy.
4. Make sure the puppy has other dogs to socialize with. Not only other puppies but mature dogs as well. They learn from both groups. Just like in their play at home with your older dog, you need to supervise their play with other dogs as well. Your puppy will need to learn proper greeting and play and will learn this best with your help.
5. Give each dog in your house it's own space, the easiest way to teach this is with a crate. Each dog should have it's own crate and it is a good idea to feed the dogs in their respective crates, especially starting out. A busy young puppy will think nothing of running over to see what the other dog has in it's bowl. Don't put the older dog in the position of defending its dinner.
Not all dogs are the same. Some have a much higher tolerance for puppy behavior, it is your job to make sure that each dog is getting the most out of this new relationship. If problems start to exhibit themselves, the assistance of an experienced dog trainer may be required.
Oh, and as to my kids, they are about 5 1/2 years apart. It was the difference between my brother and myself and it worked for us so...