Abstract Background and Aims Frailty is increasingly recognized as an important determinant of adverse outcomes in older adults with acute myocardial infarction (AMI), but its impact in younger patients remains underexplored. The aims of this study were to evaluate the association of frailty with adverse outcomes in AMI patients, stratified by age. Methods This population-based epidemiological study utilized linked national administrative data from England and Wales. Patients were stratified into three age groups: <55 years, 55–74 years, and ≥75 years. Frailty was assessed using the Secondary Care Administrative Records Frailty index with patients categorized into fit, mild, moderate, and severe groups. All-cause mortality at 1 year was the primary outcome. Secondary outcomes were cardiovascular and bleeding-related events. Results A total of 931 133 patients were included of which 13% of patients were severely frail. In patients with severe frailty, adjusted hazard ratios for all-cause mortality were 6.69 [95% confidence interval (CI) 5.76–7.76] for young patients, 4.33 (95% CI 4.11–4.57) for middle-aged patients, and 2.31 (95% CI 2.23–2.39) for older patients. The interaction between age and frailty revealed that younger patients with severe frailty had a 3.51-fold (95% CI 3.11–3.96) higher risk of all-cause mortality compared with older patients with severe frailty (P < .001). Conclusions Frailty is independently associated with poor outcomes after AMI, with the strongest impact observed in younger patients, highlighting the need for frailty assessment across all age groups.