Ageing, the phenotypic and physiological changes with age, is widespread across taxa. Yet, the patterns of ageing and lifespan vary remarkably both within and among species. Understanding the sources of this diversity remains a central challenge in evolutionary biology, particularly in natural populations exposed to fluctuating environments. In this thesis, I used exceptionally detailed long-term data from two long-lived vertebrates, the Asian elephant (Elephas maximus) and the Alpine swift (Apus melba), to investigate the determinants of ageing diversity in the wild. I first determined the age-related changes in biometric traits, reproductive traits, and parasite load in the Alpine swift, revealing clear senescence across multiple traits, but also pronounced asynchrony in the onset and rate of senescence between traits and between sexes. Then, I investigated the transgenerational effects of senescence by examining how parental age at conception shapes offspring life histories in the same population. I revealed sex-specific effects: while older mothers and fathers produced larger offspring before fledging, older fathers produced sons with higher parasite loads and shorter lifespans. Finally, I investigated the genetic basis of lifespan and survival variation in the Asian elephant population. I found substantial additive genetic variance and heritability for lifespan in both sexes, and for survival early in life (0 to 5 years old), which declined to near zero thereafter. I also found low cross-sex genetic correlation, suggesting sex-specific genetic architecture for lifespan. Across all chapters, environmental conditions emerged as a major modulator of ageing. Adverse early-life environments in Alpine swifts and spatiotemporal environmental variation in Asian elephants each explained significant variation in lifespan. Overall, this thesis provides an integrative perspective on ageing in the wild, demonstrating that ageing diversity arises from the combined influence of genetic factors, parental effects, life-history strategies, sex differences, and environmental conditions experienced across the lifespan.