AIMS: To assess functional outcomes, health-related quality-of-life (HRQoL), and satisfaction at 1-year following knee arthroplasty (KA) in patients aged ≥80 compared to those aged 65-75-years when adjusting for confounding factors. METHODS: A single-centre retrospective cohort study was performed using data from 2010 to 2020. Demographic data, length of stay (LOS), preoperative and 1-year postoperative EuroQol (EQ-5D and EQ-VAS), Oxford Knee Score (OKS) and Outcome Satisfaction (OS) were recorded. Patients aged ≥80 were compared to a control group aged 65-75-years. Regression analyses were performed to assess the independent association of age group after adjusting for confounding factors. RESULTS: Compared to the 65-75-year group (n = 7525) the ≥80-year group (n = 2966) had greater comorbidity (p <
0.001), worse preoperative OKS (p <
0.001) and EQ-VAS (p = 0.012). Postoperatively both groups had clinically and statistically significant (p <
0.001) improvements in OKS, EQ-5D and EQ-VAS. There was no difference in improvement in OKS (mean difference (p = 0.081) or OS (p = 0.203) between the groups. However, the ≥80-year group had less of an improvement in EQ-5D (p = 0.047) and EQ-VAS (p = 0.007), but these were not clinically meaningful. However, when adjusting for confounding variables there were no differences in improvement in OKS (p = 0.372), EQ-5D (p = 0.703), and EQ-VAS (p = 0.283), or OS (p = 0.829) between the groups. The ≥80-year group was independently associated with a longer LOS (95% CI 0.3-0.8, p <
0.001) when adjusting for confounding factors. CONCLUSIONS: Patients ≥80-years had clinically significant improvements in joint-specific outcome and HRQoL and OS equal to those aged 65-75-years following KA. The ≥80 group had a longer mean LOS of half a day.