Maybe the post should have asked when will lifting be beneficial for baseball? In my opinion the answer to that is when he has hair under his arms and some fuzz on his face that needs to be shaved about once a month. Then he will have enough testosterone in his system to change his body from lifting. Prior to that time he's more at risk to hurt himself, although that is always a concern with improper supervision and technique. Age 11-13 is where I saw the most injuries, and I don't think that it is a coincidence that that is when the body is changing the most. So, my advice is to take it easy during the big changes and start the lifting when the huge changes are done. Not all changes, just the huge changes.
Ughhhh. It seems as if every time I make a comment in regards to strength training, you respond with this same comment regarding "hair under arms and some fuzz on face." I am assuming you are either a doctor or nurse based on your first hand accounts of injuries. I am really not trying to be a jerk but did you review the training programs and research the background/experience of the trainer/coach who supervised the injured player? I have trained countless young athletes in and out of high school setting, many of whom did not have hair under their arms yet or facial hair, and never had someone go to the emergency room or doctor with an injury that effected playing time or lost time training. We occasionally have muscle imbalances/strains but we work directly with a local PT and in most instances are able to correct imbalances in-house.
I would also 100% disagree with your comment regarding testosterone. I would recommend that you do a little research and you will quickly find that there are numerous benefits to resistance training pre-pubescent which include strength improvements. Additional benefits include: improved bone density, motor performance, and sport performance (direct result of improved neuromuscular adaptations and motor performance).
I have witnessed the strength gains in pre-pubescent athletes which is a result of neuromuscular adaptations and does not require hormones such as testosterone. I think it is very important that we stop spreading misinformation regarding strength training for pre-pubescent children. Again, it all comes down to the quality of the training program and the individual supervising.
If you are reading this and have any concerns about strength training for your child, please...please do your homework. It is most definitely safe, effective, and has been shown to have a positive effect on performance via improved neuromuscular adaptations and motor skills. In addition, it actually helps reduce injury via improved bone density with supporting muscles, ligaments, and tendons. If you need any additional support, please message me and I can direct you to numerous studies along with anecdotal evidence from those in the know.