Background Bone-marrow water–fat composition is a useful imaging biomarker. However, the vertebral bone-marrow fat composition in healthy adults of east China is unknown. The aim of this study was to investigate differences in lumbar vertebral bone-marrow fat content between the sexes with age, using iterative decomposition of water and fat with echo asymmetry and least-squares estimation quantitation (IDEAL-IQ). Methods Three-hundred-and-twenty-one healthy volunteers (age range 20–29 years: 32/33 men/women; 30–39 years: 40/37; 40–49 years: 21/45; 50–59 years: 26/37; ≥ 60 years: 15/26) were included in the present study. All subjects underwent IDEAL-IQ sequence imaging on a 3.0-T MR scanner. Bone-marrow fat-fraction ratio (FF%) was calculated for each vertebral body from sagittal lumbar images. Men and women were compared within age-groups by independent-samples t -tests, and different age-groups and vertebral segments were compared by Bonferroni post-hoc test. FF% correlations with age and body mass index (BMI) were analyzed with Spearman’s correlation coefficient. FF% and vertebral segment correlations were assessed by partial correlation analysis, after adjustment for sex and age. Results FF% averaged over L1–L5 was significantly higher in men than in women for the < 40-year age-groups ( P = 0.000). Bone-marrow fatty conversion was accelerated in women compared to men aged 40–49 years, but was similar in women and men aged > 60 years ( P > 0.05). FF% correlation with age was weakly positive in men ( r = 0.253, P = 0.003), and moderately positive in women ( r = 0.581, P < 0.001), but was non-significantly correlated with BMI and vertebral segments ( r = 0.218; r = 0.187, respectively). FF% was higher in the lower than in the upper lumbar segments ( P < 0.05). Conclusion IDEAL-IQ can accurately quantify lumbar bone-marrow fat infiltration. This increases with age, and differs between men and women and between lower and upper lumbar segments.