Well, in my opinion there is no "too old" age, I have no problem getting an older dog from a breeder or an owner I know who has treated, socialized, and trained the dog well.

We have many fosters pass through our house, none less than 6 months old yet, and I have good faith that they will adapt to their new homes just like they adapted to my home. It's true they may take a bit longer to become friends with their new owners than a puppy, having already had an old family whom they may remember well, but still, not a huge problem. A dog has an infinite capability to love and a great wish to be accepted.