Even for someone who has never worked out, or been in the ring, 20 is a great age to start. No one can say with certainty when youth stops....your body will tell you that. It come quickly to those who sit on a couch exercising the remote control, and slowly to those who still lead a physically active life. It's not the age of 20 that holds you back, its your determination, dedication, and mind set that will hold you back. I began boxing at age 22. Went pro at 25 and boxed until age 35 (yes I got slow and tired)
I am 68. Now I train and coach boxing, and occasionally will get in the ring to do some sparring. One of my old boxing teachers was a fighter from the 1920's and 30's era. He had worked in mines, and a section hand (gandy dancer) on the railroad (carrying rails and driving spikes into the cross ties). The last time I saw him, he was in his early 70's--hard as a rock, and strong as an ox. One of the local colleges had a boxing course, and he was invited to give some training tips. The college coach made some remarks about his age, and old fashion training ideas. The old coach invited him to step in the ring to "put on a demonstration'. They went three rounds, and the story I heard, was that my old coach gave one hell of a demonstration. To his credit, the college coach apologized to him, and later told his students that he had never been hit so hard in his life, and was thankful that when the final round ended, he was still on his feet. Old Coach passed on many years ago, but he was always an inspiration to those of us who knew him...his training was hell, and he put you in the ring with brutal fighters..those of us who did not quit became good boxers. "DO NOT GO GENTLY INTO THE NIGHT".