American board of pediatrics says it's actually good for kids to lift some weights in a control environment. Teaching them the basics of proper form. Studies show boys who start lifting early in life are pound for pound stronger then other boys in high school. Builds strong bones and muscle memory so when they hit full puberty they are stronger
In general, most people (coaches, PE teachers, pediatricians) would suggest that 15 or 16 is the right tome for "weights" for boys. I've never heard anyone recommend hitting the bench press to a 12-13 year old - too much still going on in their development
That being said, strength training is important and can be done without using weights. They recommend resistance training, medicine balls, kettle bells, rowing machines, etc.
For young teens, Cross fit training is a good starter and they have teen specific programs with strength and cardio.
Couple that with speed and agility drills and kids can improve on their overall game.
although cross fit is a great exercise alternative it is not really a sports prep environment for teens your better off doing speed and agility but nothing beats swimming, biking and running
