The pathophysiological mechanism of type 2 diabetes mellitus (T2DM) is quite complicated, and its core pathogenesis is insulin resistance along with the decrease of β-cell mass (BCM) and function. Ageing will lead to a gradual decrease in the precise regulation of glucose homeostasis, and thus, the prevalence of T2DM increases with ageing. Evidence shows that ageing can lead to decreased peripheral insulin sensitivity. However, the changes in BCM and function during ageing have not been fully understood till now. The study about β-cell biology during ageing will help us to understand its contribution to age-related T2DM deeply. In this article, we review the age-related changes in β cells at the molecular, morphological and functional levels, in order to provide more aspects and therapeutic ideas for the prevention and treatment of T2DM.