The reason why some folks involve stopping a bench press movement short is because they believe that any range beyond 90 degrees during horizontal adduction/abduction can increase the risk of some kind of shoulder impingement. Let me start this by saying that the pathology of a shoulder injury is often very complex and involves several concerns (rather than just a single strain or 'weak' muscle).
Any movement you train should involve full ranges. If there is a weakness or current impinged condition, then seek help from a corrective standpoint and then resume full ROM activities. Shortening ranges, especially with young athletes, can lead to what I have heard noted as 'incomplete strength'. I kinda like this term. The body has natural ranges of motion, and to limit those ranges during strength training has a reciprocal limiting effect on performance. Also... Why the towel? If this coach is intent on limiting the natural ROM, than why not teach athletes to eccentrically control and stop the weight at a desired angle and then proceed upwards?
This is much like the 'only squat to 90' thing. First of all, at 90 degrees, the knee joint is at its most unstable position... Why stop a movement their? Secondly, your body will need to exhibit both strength and recoverability well past 90 degrees knee flexion in both sport and life. Why wouldn't you train it?
With the power lifting question - Yes, 14 year olds can certainly perform power lifts. There are however, several issues to maintain:
- Form has to be the number one concern, not load.
- Volume needs to be controlled and monitored.
- You NEED a coach with a GOOD background in teaching power lifts who can break down movements and provide appropriate feedback.
Don't let power or Olympic lifts scare you from a chronological perspective. I often teach the concepts and techniques to kids as young as 8 - 9. The nervous system is like a road map of experience - teaching concepts and movement patterns at an earlier age will only facilitate good habits in the future. ALWAYS look for technique and skill over volume and load, however.
Hope this helped!
- Brian