I have a six month old GSD puppy and I NEVER leave her with my adult dogs (five others). She is in a 4 x 4 indoor kennel during the day while I work. Some of my five other dogs are in the house in the kitchen and others are kennelled in my attached garage that is heated and air conditioned.
It only takes a moment for an adult dog to put a puppy down. Even if they did not do physical damage, having adult dogs put a puppy down repeatedly can have a vast impact upon the temperament of the puppy. I would never risk it.
Like others have said, I also have fosters coming in from time to time and integrating them in to the pack happens frequently. Keeping all of them separated when I can not supervise is key to healthy interactions amongst the dogs and preventing fights.
If you have a 10 year old dog currently, I can pretty much guarantee that the last thing that the senior dog wants is an obnoxious mouthy puppy chewing on him and jumping on him all day. Mine certainly don't and this is yet another reason to keep them separated. When all six of my dogs are together, I am right there to watch and intervene, if necessary.
Sabra - female GSD - born April 30, 2012
Sargeant - rescued senior GSD male/foster failure - approximately ten years old (August 2013)
Max - rescued GSD - Dec 22, 2010 to March 7, 2012 - at the bridge
Sable - rescued GSD - at the bridge