The purpose of this systematic review was to synthesize studies published since the last systematic review in 2015 that compare outcomes of primary total knee arthroplasty (TKA) in older patients (≥ 80 years) and in younger patients (< 80 years), in terms of complication rates and mortality. An electronic literature search was conducted using PubMed, Embase®, and Cochrane Register. Studies were included if they compared outcomes of primary TKA for osteoarthritis in patients aged 80 years and over to patients aged under 80 years, in terms of complication rates, mortality, or patient-reported outcomes (PROs). Thirteen studies were eligible. Surgical complications in older patients ranged from 0.6–21.1%, while in younger patients they ranged from 0.3–14.6%. Wound complications in older patients ranged from 0.5–20%, while in younger patients they ranged from 0.8–22.0%. Medical complications (cardiac, respiratory, thromboembolic) in older patients ranged from 0.4–17.3%, while in younger patients they ranged from 0.2–11.5%. Mortality within 90 days in older patients ranged between 0–2%, while in younger patients it ranged between 0.0–0.03%. Compared to younger patients, older patients have higher rates of surgical and medical complications, as well as higher mortality following TKA. The literature also reports greater length of stay for older patients, but inconsistent findings regarding PROs. The present findings provide surgeons and older patients with clearer updated evidence, to make informed decisions regarding TKA, considering the risks and benefits within this age group. Patients aged over 80 years should therefore not be excluded from consideration for primary TKA based on age alone. Cite this article: EFORT Open Rev 2021;6:1052-1062. DOI: 10.1302/2058-5241.6.200150