Increasing age is a commonly identified predictor of hamstring injury but is not modifiable to reduce injury risk. Why increasing age is a risk factor for hamstring injuries in athletes has not been studied to date. This study aimed to identify potentially modifiable age-related changes that predict hamstring injury in a population of Australian football players.One hundred and one young (< or =20 years), and 73 older (> or =25 years), Australian football players, without a history of hamstring injury in the past 12 months were studied prospectively. Players underwent screening of anthropometric, flexibility and lower extremity range of movement tests during the pre-season period and were followed-up for a full season with respect to injury and match participation. Comparisons of the age groups were performed to identify differences related to age. Logistic regression analysis was undertaken to determine whether the observed differences were predictors of hamstring injury.There were significant differences between the age groups with respect to body weight, body mass index, hip flexor flexibility, hip internal rotation and ankle dorsiflexion range of movement. Body weight and hip flexor flexibility were significant independent predictors of hamstring injury in players aged > or =25 years. None of the observed differences were predictors of injury in the younger age group.There are age-related changes that are potentially modifiable to reduce injury risk in older athletes and these factors should be considered in the development of hamstring injury prevention programs for this high risk group.