ObjectiveTo conduct a systematic review investigating the normal age-related changes in lung function in adults without known lung disease.DesignSystematic review.Data sourcesMEDLINE, Embase and Cumulative Index to Nursing and Allied Health Literature (CINAHL) were searched for eligible studies from inception to February 12, 2019, supplemented by manual searches of reference lists and clinical trial registries.Eligibility criteriaWe planned to include prospective cohort studies and randomised controlled trials (control arms) that measured changes in lung function over time in asymptomatic adults without known respiratory disease.Data extraction and synthesisTwo authors independently determined the eligibility of studies, extracted data and assessed the risk of bias of included studies using the modified Newcastle–Ottawa Scale.ResultsFrom 4385 records screened, we identified 16 cohort studies with 31 099 participants. All included studies demonstrated decline in lung function—forced expiratory volume in 1 s (FEV₁), forced vital capacity (FVC) and peak expiratory flow rate (PEFR) with age. In studies with longer follow-up (>10 years), rates of FEV₁decline ranged from 17.7 to 46.4 mL/year (median 22.4 mL/year). Overall, men had faster absolute rates of decline (median 43.5 mL/year) compared with women (median 30.5 mL/year). Differences in relative FEV₁change, however, were not observed between men and women. FEV₁/FVC change was reported in only one study, declining by 0.29% per year. An age-specific analysis suggested the rate of FEV₁function decline may accelerate with each decade of age.ConclusionsLung function—FEV₁, FVC and PEFR—decline with age in individuals without known lung disease. The definition of chronic airway disease may need to be reconsidered to allow for normal ageing and ensure that people likely to benefit from interventions are identified rather than healthy people who may be harmed by potential overdiagnosis and overtreatment. The first step would be to apply age, sex and ethnicity-adjusted FEV₁/FVC thresholds to the disease definition of chronic obstructive pulmonary disease.PROSPERO registration numberCRD42018087066.