Introduction

The New Dynamics of Ageing (NDA) project Healthy Ageing across the Life Course (HALCyon) responded to a growing consensus from scientists, research funders and policymakers that ageing needs to be studied from an interdisciplinary and life course perspective to inform strategies for maintaining a population that remains healthy and independent for longer.

Healthy ageing is a term that is used by many and is either undefined or has multiple meanings; this inhibits both the research and policy agendas. In HALCyon, we use the term biological ageing to capture the progressive generalised impairment of function (‘senescence’) that occurs post-maturity, caused by multiple factors, such as the growing dysregulation of homeostatic equilibrium, inflammation, oxidative stress and loss of immune function. There is a growing consensus that molecular and cellular damage that underlies biological ageing starts in utero and accumulates across life. We defined healthy biological ageing as including three components: first, survival to old age; second, delay in the onset of chronic diseases or disorders (the compression of morbidity); and third, optimal functioning for the maximal period of time, both at the individual level (measured by self-reports or objective tests of capacity to undertake the physical and mental tasks of daily living), and at the molecular, cellular and body system levels (Kuh et al, 2014b; Ferrucci et al, 2015; Ben-Shlomo et al, 2016).

HALCyon research focused on the third component of healthy biological ageing: optimal functioning. We used the terms physical and cognitive capability to describe functioning at the individual level as these terms emphasise the positive, and are distinguished from the functioning of each of the many different body systems on which capability depends (Cooper et al, 2014b; Richards et al, 2014).

Healthy ageing is also viewed, especially by older people themselves, as maintaining psychological and social wellbeing, namely how one feels and functions socially, with increasing age. Unlike physical and cognitive capability, there is little evidence for a decline in psychological and social wellbeing with age, except perhaps at the oldest ages. As evidence grows that most people age with some form of chronic disease or disorder, (Pierce et al, 2012), finding ways to support individuals or adapt the environment to maintain wellbeing gains importance.