Population ageing is one of the most profound demographic transformations of the 21st century and represents a major public health success driven by advances in medicine, public health, and socio-economic development. By 2050, the global population aged 60 years and above is projected to exceed 2.1 billion, with the most rapid increases occurring in low- and middle-income countries. However, gains in life expectancy have not been matched by equivalent improvements in healthy life expectancy, resulting in longer survival with chronic disease, disability, and functional decline. This manuscript examines the concept of healthy ageing through a public health lens, emphasizing the importance of functional ability, quality of life, and equity across the life course. Using the 6W framework, the paper explores why population ageing matters, what demographic and epidemiological transitions are occurring, who is affected, where interventions are most effective, when action is required, and how public health systems must respond. Key challenges, including multimorbidity, polypharmacy, disability, loneliness, and health system strain, are discussed alongside opportunities offered by primary health care, community-based interventions, digital health, and policy reform. The manuscript highlights the critical role of preparedness in transforming longevity into healthy ageing and underscores the need for integrated, life-course-oriented, and equity-focused public health strategies.