There is no reason you shouldn't workout before your 20's. With childhood obesity so high, it is important. it is recommended for pre-puberty kids to just use bodyweight exercises as weight could have an effect on growth. The important thing is to keep moving!
The reasons of workout vary person to person. Some people workout to gain muscular body and others workout to reduce their fat and to gain a fit body. I would like to know, what is the best age to start workout? A lot of people told me that you should start workout after 20. If you start it before 20, your normal body growth will be hammered. Please clarify.