Antiaging therapies show promise in model organism research. Translation to humans is needed to address the challenges of an aging global population. Interventions to slow human aging will need to be applied to still-young individuals. However, most human aging research examines older adults, many with chronic disease. As a result, little is known about aging in young humans. We studied aging in 954 young humans, the Dunedin Study birth cohort, tracking multiple biomarkers across three time points spanning their third and fourth decades of life. We developed and validated two methods by which aging can be measured in young adults, one cross-sectional and one longitudinal. Our longitudinal measure allows quantification of the pace of coordinated physiological deterioration across multiple organ systems (e.g., pulmonary, periodontal, cardiovascular, renal, hepatic, and immune function). We applied these methods to assess biological aging in young humans who had not yet developed age-related diseases. Young individuals of the same chronological age varied in their "biological aging" (declining integrity of multiple organ systems). Already, before midlife, individuals who were aging more rapidly were less physically able, showed cognitive decline and brain aging, selfreported worse health, and looked older. Measured biological aging in young adults can be used to identify causes of aging and evaluate rejuvenation therapies.biological aging | cognitive aging | aging | healthspan | geroscience B y 2050, the world population aged 80 y and above will more than triple, approaching 400 million individuals (1, 2). As the population ages, the global burden of disease and disability is rising (3). From the fifth decade of life, advancing age is associated with an exponential increase in burden from many different chronic conditions (Fig. 1). The most effective means to reduce disease burden and control costs is to delay this progression by extending healthspan, years of life lived free of disease and disability (4). A key to extending healthspan is addressing the problem of aging itself (5-8).At present, much research on aging is being carried out with animals and older humans. Paradoxically, these seemingly sensible strategies pose translational difficulties. The difficulty with studying aging in old humans is that many of them already have age-related diseases (9-11). Age-related changes to physiology accumulate from early life, affecting organ systems years before disease diagnosis (12-15). Thus, intervention to reverse or delay the march toward age-related diseases must be scheduled while people are still young (16). Early interventions to slow aging can be tested in model organisms (17,18). The difficulty with these nonhuman models is that they do not typically capture the complex multifactorial risks and exposures that shape human aging. Moreover, whereas animals' brief lives make it feasible to study animal aging in the laboratory, humans' lives span many years. A solution is to study human aging in the first half of ...
