Age-related cardiac fibrosis is a risk factor for heart failure and a marker for heart aging. However, the mechanisms underlying age-related cardiac fibrosis remain unclear. Ischemia and hypoxia are inevitable events in the aging process, which are direct causes of mitochondrial dysfunction and metabolic alterations. In this study, we identified metabolic changes and determined the mechanisms by which succinate (a tricarboxylic acid cycle intermediate) accumulation promotes fibroblast activation and apoptosis resistance in the aging heart. We also assessed its significance in relation to cardiac fibrosis and diastolic dysfunction.We first investigated how aging promotes persistent cardiac fibrosis. Following this, the metabolic alterations of the aging heart were identified by untargeted metabolomics, and the succinate level was verified via quantitative analysis. Furthermore, the correlations between succinate and fibrosis or diastolic dysfunction in older mice/people were assessed. We also described the role of succinate and its receptor SUCNR1/GPR91 in establishing a fibrosis network during diastolic dysfunction with age using SUCNR1-/- mouse model and an AAV9-based approach. We further identified the specific mechanisms involved in PKM2 dimerization, which regulate fibroblast activation and apoptosis resistance.We demonstrated that aging promotes fibrogenesis and diastolic dysfunction, which are linked to fibroblast activation and apoptosis resistance in the heart. Succinate levels were correlated with diastolic dysfunction and cardiac fibrosis in older mice and people. Functionally, succinate promoted fibroblast activation and apoptosis resistance, which aggravate cardiac fibrosis formation in both young and old mice. This was attributed to the ability of succinate to stimulate PKM2 dimerization via succinate receptor SUCNR1. Furthermore, dimeric PKM2 translocated to the nucleus and mitochondria, where it promoted fibroblast activation and apoptosis resistance, respectively. Accumulated PKM2 in the nucleus interacted with HIF-1α, increasing the DNA binding of HIF-1α and the expression of fibrogenic genes, resulting in fibroblast activation. In the mitochondria, accumulated PKM2 phosphorylated VDAC1 at T93 and increased VDAC1 degradation by promoting SYVN1-based E3 ligase binding to VDAC1, which is associated with increased apoptosis resistance and fibrosis persistence. We found that targeting succinate accumulation by metformin can inhibit fibroblast activation and apoptosis resistance in mice, which is a potential strategy to control age-related cardiac fibrosis and diastolic dysfunction.Our findings indicate that targeting metabolic dysregulation has significant implications for the treatment of age-related cardiac fibrosis and diastolic dysfunction. We further demonstrate a novel mechanism by which succinate induces fibroblast activation and apoptosis resistance by promoting PKM2 dimerization. Hence, inhibiting succinate generation or blocking its downstream effects is potentially a promising new strategy for slowing the heart aging.