I was wondering are boys still growing at 18 years of age?

Slowly if any. Some kids continue to grow taller into their late teens or rarely early twenties but only when their bone maturity lags behind their age. If the growth centers of the long bones have fused, you cannot grow taller. A single x-ray of the wrist ; hand can yield a bone maturity.Most boys stop at 17. When you have reached bony maturity, you are done.