I've used our weight training for players beginning in the spring of their 7th grade season. Some athletes have begun as early as spring of the 6th grade.
You need to decide when Middle School aged athletes are ready for training with weights, but all major movements can be adapted to light weight or bodyweight exercises for players who are not physically developed yet.
This program is appropriate for players from Middle School up, as long as they are physically ready for the rigors of weight training. That's a judgement call for you as a coach.
All players should consult a physician before participating in any weight training program.