This question makes me laugh. The designation of “old” and “young” souls is simply a way of communicating the amount of experience and progress a soul has made throughout its earthly incarnations. However, terms like “experience” and “progress” are ego-based and don’t easily translate into the larger scope of things. 

Time is an illusion and exists only as an agreed-upon concept by the personalities entering your reality. Truly, you’ve existed before your first incarnation into this reality. You had to exist prior to your first incarnation to even make the choice to incarnate into your reality in the first place.