OBJECTIVE: To determine the decade in which olfactory function begins to deteriorate in healthy humans by determining when odor identification is first impaired.
METHODS: Studies cited in the PubMed database were searched from its inception to March 2017 using the terms \"olfac*\" or \"smell\" and \"ag*\". The effect size of each comparison was calculated.
RESULTS: In this meta-analysis, the effect sizes as determined using Cohen\'s d for the comparisons between 30 and 39.9- and 40-49.9-year-olds was 0.06 (95% CI: -0.17 to 0.29), between 40-49.9-year-olds was 0.62 (95% CI: 0.20-1.04), considered a medium effect size, and between 35-55-year-olds and those >55years old was 1.12 (95% CI: 1.06-1.45), considered a very large effect.
CONCLUSIONS: Olfactory function deterioration, as determined by an impaired ability to identify odors, starts in the fifth decade of life in healthy humans.