Prevalence of attention-deficit/hyperactivity disorder in older adults: A systematic review and meta-analysis

•Previous studies on ADHD have mostly been conducted in children and young adults.•We provide synthesized evidence on the prevalence of ADHD in older adults.•A substantial number of older adults presents elevated levels of ADHD symptoms.•Treated ADHD is less than half as prevalent as clinically diagnosed ADHD.•More research on ADHD-clinical assessment and treatment in older adults is needed. There is a significant knowledge gap in research on Attention-Deficit/Hyperactivity Disorder (ADHD) in older adults. Via a systematic review and meta-analysis, we aimed to investigate the prevalence of ADHD in older adults, considering different assessment methods. We searched five electronic databases up to June 26, 2020. We identified 20 relevant studies with 32 datasets providing a total sample size of 20,999,871 individuals (41,420 individuals with ADHD). The pooled prevalence estimates differed significantly across assessment methods: 2.18 % (95 % CI = 1.51, 3.16) based on research diagnosis via validated scales, 0.23 % (0.12, 0.43) relying on clinical ADHD diagnosis, and 0.09 % (0.06, 0.15) based on ADHD treatment rates. Heterogeneity was significant across studies for all assessment methods. There is a considerable number of older adults with elevated levels of ADHD symptoms as determined via validated scales, and the prevalence of treated ADHD is less than half of the prevalence of clinically diagnosed ADHD. This highlights the need for increased awareness of ADHD clinical diagnosis and treatment in older adults.