The conclusion of the researchers in this study is interesting, leaning as it does towards cognitive function rather than better implementation of health practices - though both general health and cognitive function are linked via mechanisms such as blood vessel health in the brain. Untangling the knots of many interrelated correlations in such complex things as human beings isn't easy: "Conscientious individuals tend to experience a number of health benefits, not the least of which being greater longevity. However, it remains an open question as to why this link with longevity occurs. The current study tested two possible mediators (physical health and cognitive functioning) of the link between conscientiousness and longevity. ... We tested these mediators using a 10-year longitudinal sample [of 512 people], a subset of the long-running Health and Retirement Study of aging adults. Measures included an adjective-rating measure of conscientiousness, self-reported health conditions, and three measures of cognitive functioning (word recall, delayed recall, and vocabulary) included in the 1996 wave of the HRS study. ... Our results found that conscientiousness significantly predicted greater longevity, even in a model including the two proposed mediator variables, gender, age, and years of education. Moreover, cognitive functioning appears to partially mediate this relationship. ... This study replicates previous research showing that conscientious individuals tend to lead longer lives, and provides further insight into why this effect occurs."

Link: http://www.ncbi.nlm.nih.gov/pubmed/21604882