Good link/article, I pulled this from the last paragraph.
"I would recommend that juniors play with multi-filament strings that offer some “compassion” for their arms and give them some touch and feel for the ball on their strings. This will also help them developmentally as players so that they can learn about different parts of the game and develop better skills. As players mature and age up into the 16’s and 18’s poly may be an option but better would be a co-poly which is kinder and gentler than the pure poly but, still has many of the traits of the poly. I would also suggest strongly that a blend of strings (very common today) be used by juniors. An example of this would be using poly in the main strings and a good multi-filament in the crosses."