Objectives <p>To quantitatively assess lumbar paraspinal muscle degeneration and gender-related differences during aging in adults using rapid kVp switching dual-energy computed tomography (DECT).</p> Methods <p>A total of 156 healthy adults underwent lumbar DECT scans and were prospectively grouped into young (20–39 years), middle-aged (40–59 years), and elderly (60–79 years) groups. Muscle density (MD), cross-sectional area (CSA), muscle content (MC), and water content (WC) were measured using muscle-water decomposition images for the bilateral erector spinae (ES) at the L1/2 to L4/5 levels and bilateral multifidus (MF) and psoas (PS) at the L2/3 to L5/S1 levels.</p> Results <p>Across age groups, significant differences in paraspinal muscle MD and WC were observed (<i>P</i> &lt; 0.01), with MD negatively and WC positively correlated with age at lower lumbar levels for both MF and ES (<i>P</i> &lt; 0.001). In females, except for the L5/S1 PS, WC differences between the middle-aged and elderly groups were significant (<i>P</i> &lt; 0.05), but not between the young and middle-aged groups (<i>P</i> &gt; 0.05). In males, multifidus MC at L4/5 decreased with age (<i>P</i> &lt; 0.05), while in females, multifidus MC at L3/4 and L5/S1 were higher in the middle-aged group and lowest in the elderly group (<i>P</i> &lt; 0.05). PS CSA at L4/5-L5/S1 showed a moderate negative correlation with age (<i>P</i> &lt; 0.001). The Kruskal-Wallis H test and one-way ANOVA were used to compare paraspinal muscle parameters across age groups, while Spearman and Linear regression analyses assessed their associations with age.</p> Conclusions <p>The DECT muscle-water decomposition technique enables noninvasive, quantitative assessment of lumbar paraspinal muscle composition. Notably, age-related decreases in MC and increases in WC, along with gender-specific differences, may serve as early biomarkers for muscle aging and degeneration, supporting personalized evaluation and intervention.</p>