First off....I don't know many 13-14 year olds that "have tons of muscle". Secondly, how fast you develop has much to do with your physical maturaty. one 14 year old may look nothing like another 14 year old (physically) regardless of whether or not they lift a single weight. Be patient, follow a sound training program , eat right, and good things will happen.
BTW, @ 13, I wasn't even thinking about lifting weights! My biggest decision at that age was whether to ride my skateboard down to 7-11 and get the coke or a cherry slurpee......ahh, hell with it..I would just mix-em anyway!

__________________
Robert dos Remedios, MA, CSCS,
HCC (Hartman-Cosgrove Certified)
Director of Speed, Strength & Conditioning
College of the Canyons, CA
http://www.canyons.edu/departments/pe/strength
"NO CHAMPION HAS EVER ACHIEVED HIS OR HER GOAL WITHOUT SHOWING MORE DEDICATION THAN THE NEXT PERSON; MAKING MORE SACRIFICES THAN THE NEXT PERSON; WORKING HARDER, TRAINING, AND CONDITIONING HIM / HERSELF MORE THAN THE NEXT PERSON; ENJOYING HIS / HER FINAL GOAL MORE THAN THE NEXT PERSON"
-Doak Walker-