How to grow taller on age 20?

Not likely possible: If you have finished your pubertal development you have likely reached your adult height. Most people finish the vast majority of their linear growth (in height) during their mid to late teenage years. But you can always give an appearance of being taller by having good posture. Stand tall!

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. When you have reached bony maturity, you are done.