I have been extremely fortunate. My pups are all very social with other pups....they have had many come and go and they often have visitors...my friends and their pups.
I imagine part of it is because they all were rescues, so it has become commonplace for them??
I also run a tight ship. They have a schedule .. and they know what time it is and what happens at that time!
I don't leave them for long periods of time and always make sure I am on time for their meals. I think it is important for them to have structure just as it is for children to have it.
When we have a new foster home, our membership coordinator works closely with them about how to manage their fosters....she also knows their pups's personalities and tries to fit the right foster pups in their homes. Of course it is not always easy to do...because we don't know the personalities of new ones...but we do know what might not please a pup in a foster home so we try to avoid conflicts as best we can.
If you are looking to adopt, it is good to ask a lot of questions about the pup. A good rescue should keep a pup for a few weeks so that they know the personality of the pup. As for ages, yes, sometimes a young pup can drive a senior crazy.
Hope this helps.