「古い魂」is sytactically possible.
I don't know what it means by "old soul" . What is this?
I would say, giving 古い魂 lots of decorations without knowing its meaning,
These are old-fashioned buzz words. (I don't like them).
>if I wan to describe a young man with wisdom as having 古い魂をもった若者.
Then it could be「古（いにしえ）の叡知そなえし若人（わこうど）」, also a buzz expression which is often used in Japanese role-plaing games.