cocdawg:
My son pitched from both distances as a young 11 with no problems--though his travel team took the regular LL season off from tournaments to avoid the overuse/pitching availability issues. I'm sure it would have been harder to do a year earlier, but I think it also depends on the individual player to some extent.
If the mechanics and arm strength aren't yet there to be effective at the longer pitching distance (and against generally better hitters), then a pitcher trying to get something "extra" on each pitch, or going 100% on every pitch is definitely more at risk of injury. You, or an attentive coach, should be able to tell if he looks like he is straining a bit too much.
In my son's case, velocity was not an issue, so once he got more accurate at the longer distance, he was very effective at the shorter distance.