A correctly thrown curve ball is no more stress on the forarm and elbow than a fast ball. However, I never started players on throwing curve balls until they were 13/14 in the range. Depending on technique and physical attributes. The fast ball is meat and patatoes as well as change up. They should master the command on those pitches and sprinkle in some curve balls. The issue with the curve ball is MOST kids do not learn the correct way and they replace the curve with a good change up. Therefore they use the curve on almost every batter and end up throwning a lot of them during the game. I always taught my pitchers how to throw it, but spot the fast ball, get ahead of the hitter and play the ground ball with solid defense. It is okay for them to hit it, we just want to help them hit it where we want. Besides is it better to strike them out with 4 or 5 pitches or get them to hit is in 2 or 3.
The arm surgery thing is players not taking care of their arms. Long toss, bullpen sessions, icing, stretching are all very important aspects of taking care of their arm out and during the season. No matter the age. Some kids that are younger only pitch during the game with some work during practices and that does not build up strength or correct technique. Some people on here may disagree, but I have been coaching a wide variety of age groups and I have never had an arm problem.
My younger son is a sophomore now and we do a bullpen session a week, with long toss and weights the other weeks. Now as a pitcher it is also important to build you legs and core body but that would depend on the age groups.