V.B.
I can't answer whether or not you "have" to pay it off, but the bottom line is that you SHOULD. Whether it is 10 days old or 10 years old, you borrowed money from someone (even if it's a big, evil corporation or bank) and it is the right thing to do to pay it back if you have the means. Even if you have to do it over a period of time, you owe them the money, plain and simple.
So, while there may be a statute of limitations by law that doesn't require you to pay it back, my conscience wouldn't allow me to ignore it knowing that I received benefit from it. Good luck!