If he can compete - up. If he is a slow grower and would be better suited catching up to his peers - his age.
Both of mine were playing varsity as a freshman. One a two way, the other a pitcher who hit on occasion. They both got the start on the mound in the state semis their freshman years. What good was playing 14/15u going to do them?
The summer following freshman year. One played 17u as a two way. The other pitched on his 17u team as a PO and for his 15u team as a SS. Same org so they were able to coordinate.
Once you are prepared to try out for the varsity team age no longer matters.