Old Soul

What is an "old soul"? The context in which I hear it indicates that it means "simpleton".
I thought that if someone has an old soul, they are wise beyond their years. Obviously,
a conflict here.

I think it's a term of praise among believers in reincarnation: having an old soul means you've been around several times and you've accumulated a heap of good karma. (If you'd been around several times heaping up bad karma, you'd be here as a dandelion or something.) But that's probably not the only way it's used.