Introduction: Ageing and physical inactivity are independent risk factors for musculoskeletal decline. The aim of the current study was to assess the influence of age on the musculoskeletal changes incurred during short-term physical inactivity. Methods: 26 young (28 ± 8 y) and 21 older (60 ± 6 y) habitually active (>10,000 steps per day) participants were recruited. Musculoskeletal assessments, including dual-energy x-ray absorptiometry to quantify lean mass and bone mineral density (BMD), magnetic resonance spectroscopy to quantify mitochondrial function and intramuscular adipose tissue (IMAT) and a leg press protocol to assess muscle function/strength were performed at baseline, following 14 days of step reduction (~1,500 steps per day) and after 14 days of returning to habitual activity. Statistical analysis was performed using two-way repeated measures ANCOVA, and data are presented as mean±SD. Results: Average daily step count reduced by 10752 ± 2943 steps/day in the young (83 ± 7%) and 10204 ± 2338 steps/day in the older group (84 ± 7%), with concomitant increases in sedentary time (P<0.001). The intervention induced significant losses in total body lean mass (Young 0.43±0.78kg vs. older 0.08±0.76kg; P=0.024;), lean leg mass (young 0.18±0.33kg vs. older 0.08±0.33kg; P=0.003) and muscle strength per unit of lean muscle mass (muscle quality) (young -0.33±0.92kg vs. older 0.33±0.51kg; P<0.001), which were not significantly different between age groups (P>0.05). Correspondingly, IMAT increased (young 0.67±2.84% vs. older 2.73±3.97%; P=0.04) with no difference between groups (P=0.27). Serum markers of bone resorption (CTX) were elevated (P=0.002) in response to the inactivity period to similar extents in both age groups (P=0.54). Cardiorespiratory fitness and mitochondrial function significantly declined during the inactivity period, indicating reductions in whole body (VO2 peak: young 1.99±3.68ml.min.kg -1 vs. older 2.75±4.12ml.min.kg -1; P<0.001) and skeletal muscle oxidative capacity (rate constant: young -0.08 ± 0.39ks-1 vs. older 0.38±0.36ks-1; P=0.007). Importantly, these reductions were significantly greater in older adults (P>0.05 ). Conclusion: Results suggest that even short-term modest reductions in physical inactivity induces substantial impairments in musculoskeletal decline, regardless of age. Reductions in oxidative capacity were more pronounced within older adults indicating an increased vulnerability to physical inactivity compared with young. This highlights the importance of maintaining levels of physical activity throughout the lifespan.