Times have changed. We now think of bodybuilding as a sport like any other, and safer on the body and mind than most team sports, especially football. Still, most adolescents don't have the time, money, or endurance for daily exhaustion sets with 50-pound dumbbells, so physiques like this are rare.
But a lot of 15- and 16-year olds have obviously trained physiques.
(Photos taken from youtube. For comparison purposes only.)
Others, not so obvious, until they are convinced to flex for the camera.
13- and 14-year old bodybuilders are rare. You're still at the start of puberty, with vast changes in height and weight, metabolism, and hormones. It's hard to train muscles with all that going on. But a few kids have the right combination of genetics and dedication to attract the attention of all of the boys in their class.
According to his youtube channel, this is 13-year old bodybuilder Brad the Beast, who has won several state and regional powerlifting competitions.