I feel very sad for you since you are harbouring all wrong notions regarding masturbation
The fact is that masturbation and sexual activity in general have no significant effect on testosterone levels. Testosterone levels involve several factors, such as age, time of day and physical activity.
As far as growth plates are concerned,they have no relation with your masturbation.
ALL of us, stop growing around age 18-21. I understand your concern about your height. Our society is obsessed with men who are tall. From actors, to super heroes, musicians, business people, etc. All are looked upon a successful when tall, less successful when of shorter height.
To simply say; do not worry about your height issues, or embrace your height issues, would be an insult to you! Self-esteem has to be built, not thought. The best way you can achieve unbreakable self-esteem is to join a gym and began a lifetime program of exercises. It does not have to be strenuous as bodybuilding. Simply strength training. Weightlifting will make you stronger, more massive, than any one else around you. Having strength gives you amazing posture, endless self-confidence and admiration of others; no matter what your height.
It is less about growth plates and more about genetics. If you have tall parents then it is likely you to will be tall. You can also be taller if you eat healthily and dont smoke and do regular exercises,as you still have few years to add few inches to your height.
Enjoy your life and for god sake don't mess it up.