I'm at a total loss as to why anyone would think karate or any other martial art would stagnate growth. Unless it is run in the form of an ultimate fighting tournament and results in major damage to the person. Martial arts taught by a competent individual should encourage growth.
Use of the muscles and tendons, increased flexibility and improved circulation should all benefit the practitioner and cause them to grow and develop muscles and stronger bones.
Karate training cannot be done in a manner that will hinder growth.