In a study of 13,451 elderly patients, being obese after the age of 75 years did not carry the increased risk of death that was seen in people who were obese when young.
Among those aged more than 80 years who were overweight, the risk of all-cause mortality was lower than that of people with a normal weight.
Lead researcher Dr Maria Corrada, from the University of California at Irvine, said: 'It seems if you're in your eighties or nineties, you may live longer if you are a bit overweight by BMI standards.
'We found that what's recommended for everyone else with BMI measurements isn't necessarily best for the elderly.'
Participants, aged on average 73 years at the start of the study, were asked for their height and weight at age 21 and at the time of the survey. In 23 years of follow-up, 11,203 died.
Compared with being a normal weight on BMI, being underweight at any age was associated with at least a one-third-higher risk of mortality.
Being obese when young was also linked with a significantly higher risk of death. But people who were a normal weight when young and gained weight as they aged were not at a higher risk of death than those of normal weight at 75 years or more.
Among those aged 80 or over, the risk of death was 10 per cent lower in those people classed as overweight.
National Obesity Forum clinical director Dr David Haslam said this could be because obese patients who reached that age probably did not have the metabolic syndrome. But he added: 'GPs should treat obese elderly patients the same as young people and check for comorbidities.'
Am J Epidemiol 2006; 163: 938-49; Live links at GPonline.com.