The metabolic cost of walking reflects gait efficiency and is influenced by biomechanical factors as walking speed. While most walking research uses treadmills, older adults are found to exhibit a greater elevation in the cost of walking on the treadmill compared to overground walking than younger adults. A possible cause for this elevation could be the higher stability demands in older adults during treadmill walking, possibly influenced by a higher vestibular contribution. This study investigated whether increased vestibular demands for balance control contribute to this elevated cost in older adults. Ten younger (mean age 26.4 years) and ten older adults (mean age 68.6 years) completed 5-minute treadmill and overground walking trials at preferred and slow fixed speeds. Metabolic cost was measured, and vestibular contributions to balance were assessed via electrical vestibular stimulation, which induced virtual movements and evoked balance correcting responses measured by inertial sensors on the back and ankles. Treadmill walking increased the cost of walking significantly by 15-23% compared to overground walking, with no significant age effect. Vestibular stimulation increased metabolic cost significantly in both overground and treadmill walking and age groups. Assessment of the vestibular contributions to kinematic measures revealed a significant increase in vestibular contribution to balance at slower walking speeds, but no significant effect of age and no large effect of treadmill or overground. Indicating that the measured participants cannot conclude that the elevation of cost in treadmill walking in older adults is due to the vestibular contribution to balance.