A spiritual person whom is wise beyond their years; people of strong emotional stability. Basically, someone whom has more understanding of the world around them.
Some people even believe an old soul is a person whom has learned from past incarnations, or lives. They aquired certain knowledge from their past lives and apply it to their present life... thus gaining more wisdom than the average bloke.
"She has an old soul."
Word used to describe to sickened friends the underage girl you're trying to have sex with.
"I know jenny's only sixteen, but she's a really old soul."
"Thanks for coming up to see my paintings, have some wine... Oh! you don't look that young, I think you're beautiful and you're such an OLD SOUL!"