Different perspective: Randy Sullivan at FBR is a Master Physical Therapist, Pitching Coach and a pretty good guy to see if you are experiencing arm pain. When my son was 15, I asked Randy what his thoughts where on having him start wieghted balls..... he was quick to say not until he felt that my son was physically mature enough, had demonstrate a certain level of profciency with his mechanics, strength to support the associated stress, as well as, what he considered an acceptable level of mobility and flexibility.
In Janaury of 2018 my son (17 years old) will incorporate a less intense version of a "standard" wieghted ball, long toss and wieght lifting program into his reqular FBR routine as I am not yet sold on (overly cautious) weighted balls, long toss and wieght lifting. My stance on being overly cautious is driven by the fact he went from sitting 72-73 and touching 74 to sitting 85-89 and touching (90 at PG) and (91 at UA/BF) in less than 2.5 years while training at FBR....all without wieght lifting, using wieghted balls or long tossing. My son is a 2019 LHP 5' 10" @ 185lbs.