Do you think it's common for people to cash in old bills at the bank or do you think most people are wise enough to keep them?
First of all, just because bills are old does not mean that they are valuable or worth more than face value
Secondly, everyone is in a different situation and they may have no option other than cash the notes.
Thirdly, I know for a fact that I have seen many old bills (coins) coming from a lawyers office where estates are being closed and their interest is not in the old bills, just to get the cash in the bank.
I am sure there are many other instances/reasons why people return notes....