It depends on the scout. I am the SPL of my Troop currently (my term ends in a week!), and I was elected twice. So I started the summer before 9th grade, and I did not feel like I was ready for it. Turned out that I underestimated myself and I could do it. There are no requirements but it depends on the scout.
If your scout feels like he can do it, then he can do it.