OBJECTIVE: The aim of this study was to elucidate whether age plays a role in the expansion or regeneration of beta-cell mass. RESEARCH DESIGN AND METHODS: We analyzed the capacity of beta-cell expansion in 1.5- and 8-month-old mice in response to a high-fat diet, after short-term treatment with the glucagon-like peptide 1 (GLP-1) analog exendin-4, or after streptozotocin (STZ) administration. RESULTS: Young mice responded to high-fat diet by increasing beta-cell mass and beta-cell proliferation and maintaining normoglycemia. Old mice, by contrast, did not display any increases in beta-cell mass or beta-cell proliferation in response to high-fat diet and became diabetic. To further assess the plasticity of beta-cell mass with respect to age, young and old mice were injected with a single dose of STZ, and beta-cell proliferation was analyzed to assess the regeneration of beta-cells. We observed a fourfold increase in beta-cell proliferation in young mice after STZ administration, whereas no changes in beta-cell proliferation were observed in older mice. The capacity to expand beta-cell mass in response to short-term treatment with the GLP-1 analog exendin-4 also declined with age. The ability of beta-cell mass to expand was correlated with higher levels of Bmi1, a polycomb group protein that is known to regulate the Ink4a locus, and decreased levels of p16(Ink4a)expression in the beta-cells. Young Bmi1(-/-) mice that prematurely upregulate p16(Ink4a)failed to expand beta-cell mass in response to exendin-4, indicating that p16(Ink4a)levels are a critical determinant of beta-cell mass expansion. CONCLUSIONS: beta-Cell proliferation and the capacity of beta-cells to regenerate declines with age and is regulated by the Bmi1/p16(Ink4a)pathway.