Frailty: understanding the difference between age and ageing.

In the past, illness and dependence were viewed as inevitable consequences of old age. Now, we understand that there is a difference between age (the passing of chronological time) and ageing (the increased risk of adverse outcomes over time). Over the last 50 years, 'frailty' research has established that ageing is heterogeneous, variable and malleable. Significant advances have been made in frailty measurement (description of clinical features and development of clinical models), mechanisms (insights into pathogenesis) and management (development of interventions to reduce and/or prevent progression). Subsequently, the concept of frailty has informed health policy and clinical practice and started to change perceptions of older age held by the general public and the health sector. Here, we overview key achievements in frailty research and clinical practice and highlight the considerable number of known unknowns that may be addressed in the future.