This ties in with a belief in reincarnation, such as is held by several oriental religions. The idea is that, on death, you reappear as someone/thing else in order to try to correct the flaws in your soul that you didn't manage to get rid of during the life just ended. Absolute perfection is the ultimate aim. If this were true, then obviously lots of us - if not all of us - would be wandering about with "old souls". Generally, it is said of someone relatively young who has ideas shared with older people rather than those of his/her own generation. That is, they are wise beyond their years.