In reply to elsewhere:
As a life-long Scouter, I obviously agree with the idea that Scouting and Guiding is good for you. However, as I scientist, I do wonder whether this shows that Scouting leads to long-term mental health, or those who will have better long-term mental health are more likely to join the Scouts.
The first hypothesis means that the 10 year old's brain is still developing, and it is decisions after this age which determines the mental health of the person: we (society) are at least partially responsible for the mental health problems we see because of the way we treat children.
The second hypothesis means that the 10 year old has already got long-term mental health problems 'built in', and little that we can do will prevent it: all we can do is treat the symptoms. Of course, if we can identify the signs in 10 year olds, we can start treatment earlier.
Im not sure which hypothesis I would prefer to be true...