Age‐related neural and musculoskeletal declines affect mobility and the quality of life of older adults. To date, the mechanisms underlying reduced walking economy in older adults still remain elusive. In this study, we wanted to investigate which biomechanical factors were associated with the higher energy cost of walking in older compared with young adults. Fourteen younger (24 ± 2 years) and fourteen older (74 ± 4 years) adults were tested. Plantarflexor strength and Achilles tendon stiffness were evaluated during a dynamometer test. Medial gastrocnemius fascicle length, ground reaction forces, joint kinematics, and oxygen consumption were measured during walking treadmill at 0.83 and 1.39 m.s−1. Energy cost of walking, lower‐limb joint mechanics, muscle‐tendon unit, and tendinous tissues length were calculated. The energy cost of walking was higher at 0.83 m.s‐1 (+16%; P = .005) and plantarflexor strength lower (−31%; P = .007) in older adults. Achilles tendon stiffness and medial gastrocnemius fascicle length changes did not differ between older and young adults. The reduction in ankle mechanics was compensated by increases in hip mechanics in older adults during walking. The hip extensor moment was the only significant predictor of the energy cost of walking (adjusted R2: 0.35‐0.38). The higher energy cost in older adults is mainly associated with their distal‐to‐proximal redistribution of joint mechanics during walking possibly due to plantarflexor weakness. In our study, medial gastrocnemius fascicle and tendinous tissue behavior did not explain the higher energy cost of walking in older compared to young adults.