So what happens if you recently answer an old question, and the questioner is no longer around to accept it. After a certain time or vote received threshold is it auto accepted?
No. Why would it be "auto accepted"? "Accepted" status tells readers that the author of the question decided to select this answer as the most helpful - nothing more, nothing less.
From the help center:
Accepting an answer is not meant to be a definitive and final statement indicating that the question has now been answered perfectly. It simply means that the author received an answer that worked for him or her personally. Not every user comes back to accept an answer, and of those who do, they might not change the accepted answer even if a newer, better answer comes along later.