I skimmed the article that MON linked. From what I gathered, early school entrants had higher rates of alcohol use and lower psychological adjustment at middle age. These factors partially accounted for the increased overall mortality rates. The paper didn't mention anything about exercise.
What I found interesting is that being out-of-sync with age peers in 8th grade was not a predictor of poor outcomes later on. So I guess it would be safer to put gifted kids in first grade no earlier than 6, but then grade skip later as needed.
The weirdest thing was that "conscientious kids" had better outcomes than "cheerful kids". I would have thought the conscientious kids would be more likely to burn out. But I guess it depends on how they coded kids as conscientious vs cheerful.
