Dec 23, - Most teens don’t have any reason to try to gain any weight. Teens – especially guys – may expect to develop some extra muscle weight as they grow into their adult bodies. Begin strength training by doing only body weight exercises such as sit-ups and pull-ups prior to lifting. Aug 6, - Read about Mukul's journey from a super-skinny teen to a fit, athletic adult. I've always been an avid sports enthusiast. I participated in athletics.