Macassie, I'm so sorry.
It could just be a coincidence that all the birds died, but there could also be some environmental factor. For example, a space heater being used (some of them have teflon coating on the coils), nonstick cookware, something cooking off the oven or furnace, some pathogen on the cage from before, some cleaning product...even things like radon gas or carbon monoxide, things that might not be noticable to humans but birds are so much more sensitive. Is there a water softener...any old toys or perches that might be harboring something toxic...sometimes homes have been sprayed in the past with what are now banned pesticides or fungicides...are you using any foods left over, say in the freezer, from before that might be contaminated...there are so many different things. If the vet can determine a cause it would be reassuring, because then at least you could deal with the situation.
As far as young vs. older birds...a young bird is sweet, you can have a big hand in their socialization, but when they hit puberty they can turn into monsters and have complete personality changes. Older birds, they may have baggage from poor treatment or they may be total sweethearts, but you usually get a better sense of their personality, and they may already have changed into whatever they are going to be. Many times a parrot winds up in a rescue or shelter because the first owner wasn't prepared for the responsibility and "burden" of a parrot, and they might have no issues at all. Some of the most difficult birds, over time, can become the sweetest. It's like adopting a random kid from the orphanage, you have to just take your best guess and go in with all the love you have.