Welcome to Health Care Magic.
You need not worry.You are absolutely normal and nothing is wrong with you. Masturbation
does not effect the growth of beard or moustaches.
Delay In growth of Beard and moustaches is a common problem. I guess you have beard on the sides. It would take sometime for the beard to grow especially under the chin. You need stop masturbating.
Nothing is wrong with you. You are just 18 years. Do not worry much about these small things. With in 2-3 years your beard should grow.
Wish you a healthy life.