In this recent article published in the Journal of the American
Geriatrics Society the authors show that not being sedentary is more
important that weight for elderly individuals aged 70-75.
Interestingly, people who were classified as overweight had the lowest
relative risk of death. The authors conclude, "Overweight older people
are not at greater mortality risk, and there is little age that dieting
in this age group confers any benefit; these findings are consistent
with the hypothesis that weight loss is harmful."