Tons of young people still play sports and are physically active. Maybe it is just the bubble I live in, but kids who are on even rec leagues from 5 years old on are engaged in organized sport activity multiple nights a week.
If you start out reasonably fit it takes less time. If you are heavier you need a longer runway.
Maybe a pre-bootcamp for large people where they introduce basic nutrition and basic exercise. That might actually be a big draw for obese people who want to turn their lives around.
If you didn't learn this in grade school it might already be too late.
This [0] was linked up-thread and touches on increasing injury rates that the US army has worked to get down