Age-related changes in performance (reproduction and survival) are widespread in nature. In vertebrates, age-related performance trajectories are typically characterised by early-life improvements followed by late-life declines (termed ‘senescence’). However, the mechanisms that underpin such age-related changes in performance are still poorly understood. In this thesis, I combine molecular, cellular, long-term life history, and radio-telemetry data from a population of a cooperatively breeding bird, the white-browed sparrow weaver Plocepasser mahali, to investigate some key shortfalls in our understanding of the mechanistic origins of age-related changes in organismal performance. In Chapter 2, I present the first longitudinal evidence that within-parent changes in age positively predict offspring telomere lengths, an effect that has the potential to positively impact parent and offspring fitness. Then, in Chapter 3, I present the first evidence for within-individual accumulation of senescent cells with advancing age in a natural population. I also demonstrate that these within-individual increases in senescent cell frequency over the short term are not reflected at the population level, suggestive of the selective disappearance of either individuals with high incidences of senescent cells from the population, or of senescent cells within individuals. In Chapter 4, I investigate the effects of age on dominance acquisition, and provide evidence that individuals that are older relative to other potential competitors are more likely to win dominance turnovers. This finding highlights a potentially widespread mechanism by which the costs of cooperation (in terms of increased competition for breeding positions) could be mitigated in cooperatively breeding species. Finally, in Chapter 5, I investigate whether within-group social circumstances influence subordinate dispersal behaviour. Despite finding that individuals do not seem to adjust their dispersal behaviour in response to the expected payoffs of remaining in their current group versus dispersing, I show that individuals tend to disperse to subordinate positions in smaller groups and with fewer same age or older individuals. As living in a larger group entails survival costs, and individuals are more likely to win dominance in groups where they are the oldest subordinate of their sex, these results suggests individuals may ‘trade up’ their social circumstances when they disperse. Together, my findings elucidate some key shortfalls in our understanding of the mechanisms underlying age-related changes in performance.BBSRC