I m boy now i m 21 years old i m 5.2 ft tall.......

What is the cause of your current height ?
your current height is considered normal and caused by genetic accomulations and effects , and not cause by disease or a medical condition 
Is it possible to increase height at 21 years old :
it may be still possible to increase your height and stimulate your bones growth in some medical ways in this age , but first you have to consult an orthopedic doctor and do an X-ray test to see if there is still the layer which is still responsible to increase height , which usually disppear around your age 
How to increase height at age 21 :
1. practice jumping and running sports such as basketball
2. avoiding stress 
3. have plenty of rest and regular sleep
4. using legs or lower weights during the day
5. using some vitamins suppliments after consulting your doctor 
What is the helpful vitamins and minerals that help you grow taller :
1. vitamin A
2. vitamin E
3. vitamin B1
4. vitamin D
5. vitamin C 
6. zinc