I am Ravikumar, 21 years old, interested to become a boxer. How many years does it takes to become a pro if I start now and is this the correct age to start?

Yes you can start it depends on you how you train yourself you can participate  in amature boxing competitions of senior level till the age of 40 on the other side there is no age critiria for professional boxing ...it will be good for you participate in amature boxing competition for experience  and as well as for your profile as a boxer to start professional boxing.   

Hello Mr. Ravi , Any person physically fit and between the ages of 18 and 36 can become a boxer. However, the preparation period is long and quite demanding. It includes daily training to condition the body and learn the techniques necessary for the sport. Motivated candidates can eventually become professionals and gain the fame and fortune that comes with winning matches. They say it takes at least 3 years in the making of a boxer. That said, you could become a fully trained boxer by the age of 24 05 25.