Longitudinal studies of ageing: from insights to impacts: commentary to accompany themed collection on longitudinal studies

'Time is the best diagnostician': who has not thought this? In clinical practice, presentations are often subtle and decisions made in the face of a 'snapshot.' Crystal balls do not exist; yet, insights from longitudinal studies can help to recognise emerging pictures and anticipate typical trajectories. In the multifactorial, biopsychosocial world of geriatrics, the determinants of those trajectories, and hence opportunities to modify them, can be better understood through careful longitudinal disentangling of the wider determinants of health, and this can be done at multiple levels of analysis, from molecules to society. With this collection and commentary, we highlight the approaches, scope and impacts of a selection of longitudinal studies of ageing published in Age and Ageing within the past 10 years. Longitudinal studies can illuminate disease mechanisms, how declines in multiple domains of intrinsic capacity interact, how losses in one domain may influence the path of another, and in turn, how these changes translate to functional disability, or not. Observing trajectories of geriatric syndromes can suggest opportunities for optimisation and prevention in clinical practice and policy. With global opportunities for harmonising data, longitudinal studies are already offering the opportunity for cross-national comparisons and for developing hypotheses about the relative contributions of time, place and society in the trajectories of frailty, disability and quality of life. We also include studies which show how research-based longitudinal data can be synthesised or be linked to administrative datasets. We hope you find this collection as interesting and encouraging as we have.