I started at 14 with serious bodyweight exercises and laid a good foundation. The weights were intorduced at 15 and reps were very high - certainly never lower than 20 until I was 16. Then I still did moderate reps like 15, 12, 10. When I finished my third year of resistance training I introduced lower reps and became more interested in really adding mass and strength (age 17).
I was very healthy during this entire time, and felt more confident and got respect at school and grew to be two inches taller than my Dad who is 5'10" (so I dont believe that it stunted my growth).
I would reccomend that plenty of sports are played, and that weights should not really be the sole persuit of a teenager, rather the core of a diverse exercise program.
SWIMMING is terrific for any age, but will assist the teens in staying limber and maintain healthy joint function. 1 -1.5 miles a week split between 2 sessions should be enough.