IMO, it's not so much the age/injury question, but rather the age/mechanics/ability question. Injuries occur more often because of poor training/bad mechanics than just because of age. However, trying to teach a 10 yr. old to throw a rise is like trying to teach them to ride a Harley - a little too much too early.
In simple terms, the successful pitchers have figured out what it takes to get outs (not necessarily strike-outs) at their age-appropriate level. At 10u, that's usually just being able to "throw strikes". Wild junk at a young age does nothing more than help the opposing team. Certain pitches require a level of strength and physical development. While a fastball with an upward path is usually very effective, it's not going to have enough spin to be a true rise ball. And in the process, the kid will probably be somewhat wild with it, and wind up with a sore forearm.
Notice that the best hitting instructors (Howard C. for one
) have a net with markings that indicate the ball trajectory. Not only are hitters taught how to "hit balls", and know their own strike zone, they are being taught how to hit to all parts of the field. If a pitcher is just "throwing" with no real sense of control, not only will they get hammered by these well taught hitters, but they're also in real danger of being injured! That's my case for "keeping it simple" and focusing on fundamental mechanics, which produces speed AND control. Only then should the "junk" pitches be put into the mix.