To which age still a boy or man continue developing . i mean in terms of being taller . to the age of 21 years old ?

It's depends....: ...On when a boy starts puberty. Those who start at 10 years old are usually done growing by 15. If he does not start puberty until 16, he may grow until 21. Average age on onset of puberty in the us is 11.6 and average age to adult height is between 16-17.

Unlikely to happen: 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.