Omni, channeled by John Payne:

When we refer to young soul, mature soul, old soul, etc., we are not talking about age. We are talking about experience. Although in the majority's experience, 'old' souls have had more lifetimes than 'young' or 'baby' souls, it is possible for a 'mature' soul to have lived many more lives than an 'old' soul.

So what do we mean when we speak of soul ages? We refer to experience, and this experience is about becoming increasingly aware of the total self whilst in the physical state. Those of you who are seekers of truth and of self are responding to the impulses of your Greater Self, your Inner Being, your Soul, for it desires that you should know it, and it desires to know itself through you. Your Inner Self desires to be fully conscious through you, while being at one with the All-That-Is. Soul age is determined by the individual soul's level of skill at being consciously present in the physical plane.

^Source Omni Reveals The Four Principles of Creation by John Payne

If you are considered an older soul on earth, you are simply more experienced on the earth plane. A younger soul on the earth plane could be more experienced on another planet or realm but newer to earth. Utimately, all souls are equal.