I know there are lots of opinions out there regarding this subject, and unfortunately all I can offer is my opinion. I started throwing the curveball when I was about 9, the first year I started pitching. I never really had any arm trouble until I was 24 and in my fourth year of professional baseball, so I'm pretty confident throwing the curveball at a young age didn't cause any long term effect on my arm. As for your son, I don't know. Maybe I didn't get hurt because I had good mechanics with it.
Bad mechanics are definitely a major factor in any arm injury at any age.
I'm sure growth spurts and physical development factor in as well. We're glad everyone's having fun. Now for the other part. A lot of pitching coaches get really active in working with a pitcher's mechanics. I was a first-rounder in '93 and had more advice and instruction given to me in two years than some have in their whole careers.