http://www.bodybuilding.com/fun/drobson38.htm
Good article.
As your body grows through-out your teens you have a big opportunity to set your body up for the rest of your life. Proper form and strengthened muscles can ensure proper posture, and prevent the hundreds of complications adults deal with later on in life. Bad back, over-used hips, knees, ankles, etc...
However, the very reason it can be very beneficial to train at a young age is the same reason its very risky. Your body is still growing. Bad form, or an injury can severely make your adult life more complicated. If you could hunt down some of the people I played football with- ask them how their ankle is holding up with 3 pins in it, or whether they can still run with a torn ACL.
Obviously more risk in football but their is risk in weight training, and its greater at a younger age. So make sure you're doing it RIGHT. A personal trainer, phys ed teacher, coach, etc can show you proper form. Don't "wing it".
And now that we're done with getting side tracked on "weight training"...
What you really want to do is cardio to get rid of body fat.
The actual loss of body fat can not affect your growth. How you lose it may, but not by doing cardio/aerobic exercise.