Evidence suggests that those with a Body Mass Index (BMI) greater than 25 are more likely to die younger due to diseases such as cardiovascular disease, cancer and diabetes, but like most things - there are exceptions. Convincing evidence suggests that (on average) a higher BMI increases the risk of chronic disease and mortality, however, elite athletes and people with higher lean body mass (e.g. weight lifters) may have a higher BMI despite a lower cardiovascular risk conferred by the effects of regular exercise, and in many cases - healthier eating.
A healthy BMI range may be higher for populations such as older people, those of Polynesian origin, and athletic individuals. Conversely, the healthy BMI range may be lower for people of Asian descent.
Despite these exceptions, BMI remains a reliable and modifiable health risk factor for the general population and should be used as a guide to monitor a healthy weight range for most people. Most adults should therefore aim for a BMI in the 'healthy' range of 18.5 - 24.9. For the exceptions listed above, your doctor might look at other important assessments of health, including waist circumference, body fat composition, blood pressure and blood cholesterol.
Australian Government - Department of Health