The study aim was to comprehensively assess physiological function and muscle morphology in chronically trained older individuals against untrained young and older individuals. In a cross-sectional design, 15 young untrained controls (YC) (20 ± 2.7 years, 78.9 ± 13.3 kg), 12 untrained older controls (OC) (69.8 ± 4.1 years, 77.5 ± 14.2 kg), and 14 endurance-trained master athletes (MA) (67.1 ± 4.1 years, 68.7 ± 6.5 kg) underwent assessments of body composition, aerobic capacity, strength, muscle architecture, and fiber-type morphology. Skeletal muscle index was lower and body fat greater in OC versus MA. Estimated VO2max (mL·kg−1·minute−1) was similar between MA and YC, but lower in OC. Isometric leg strength normalized to fat-free mass was similar between groups, whereas normalized isometric arm strength was greater in YC than MA. Myosin heavy chain (MHC) I fiber area was greater in MA than OC, while MHC II fiber area was greater in YC than OC. MHC II fiber myonuclear domain size was greater in YC than OC and MA, whereas MA had greater MHC I and MHC II fiber capillarization than OC and YC. Satellite cell content was similar between groups. Chronic endurance training enhances indices of muscle morphology and improves body composition and aerobic capacity in older age, with potentially important implications for healthspan extension.