Many people think of physical activity as a bonus: adding physical activity can help health, but rarely expect that a lack of physical activity is harmful. For instance, the studies in children which show declines in BMI from decreasing screen time are all mediated by food intake, rather than exercise: screen time seems to increase BMI by increasing food intake, rather than by decreasing activity.
Apparently, decreasing physical activity is bad for health in measurable ways for this n=18 study conducted over 3 weeks: increased insulin, triglycerides, intra-abdominal fat mass, C-reactive protein, and decreased lean mass.
They "developed metabolic changes suggestive of decreased insulin sensitivity and attenuation of postprandial lipid metabolism and physical changes that suggest that calories used to maintain muscle mass with greater stepping may have been partitioned to visceral fat."
Not specific to adolescents, but nonetheless interesting!