Teenage boys in this modern society need to exercise to build muscle - their bodies expect it and were intended for it at that age, so they would start adulthood more fit.
In long-forgotten times, teenage boys WERE more physically active, because life and “life style” REQUIRED it - they worked, in some capacity or another, and the further back in time you go, the deeper into the long-ago establishment of the genetic pattern you go, the work was more physically demanding in one way or another.
Todays society, to the extent it discourages teenage boys NOT to give their bodies muscle building, muscle toning exercise, is asking the boys to (a)ignore what their bodies expect at their age (their metabolism is primed for it) and (b) forgoe good habits that will serve them well in their teens, good habits they will be more inclined to keep - in some fashion - when they are older, if they are a regular part of life already.
Is it necessary to seek to become a “Charles Atlas”? No. And the survey statistics indicate that only a small percentage are trying to do that.
Indeed, and some still do. This reporter needs to visit a farm and see what it's like for a 12 year old boy to work 12 hours a day up in a 100 degree barn throwing and stacking hay bails. I think I bailed hay every summer from when I was 9 to 20 years old. No gloves, no shirt, no long pants. $2/hr. Glad to get it.