The WHO defines health as “a state of complete physical, mental, and social well-being, but not merely as the absence of disease or infirmity”. Ageing starts before birth and depends on factors that operate throughout life, and hence, the concept of healthy ageing also encompasses healthy growing and healthy living. Indeed, disorders in adults often find their origin in risk factors operative in early life. Senescence or biological ageing is the endogenous process of cumulative changes to molecular and cellular structures leading to disrupting metabolism with time. We study over the first two decades of life how genetic, environmental, nutritional and lifestyle factors, and their interactions, via telomere “interactome” differentiate healthy from unhealthy ageing. In line with the WHO definition of health, differentiation between healthy and unhealthy ageing / growing should start before progression to overt disease, when prevention or intervention still remain possible. The proposal builds on existing cohorts in newborns and children and adolescents, who underwent extensive high-fidelity phenotyping with available biobanks and longitudinal follow-up.