With age, loss of skeletal muscle mass (sarcopenia) results in decreased muscle strength and power. Decreased strength and power, in turn, are closely linked with declines in physical function. Preferred walking speed, a marker of physical function, is slower in older adults compared to young adults. Research suggests that older adults may walk slower as a consequence of decreased plantarflexor power at push-off. In this study, we hypothesized that providing additional plantarflexion (PF) power during push-off would (1) increase preferred walking speed, and (2) reduce metabolic cost of transport (MCOT), in young and older adults. PF power was augmented using powered ankle-foot orthoses (PAFOs). The PAFOs, which use pneumatic actuators to provide an additional PF moment, were based on a design by Ferris et al. [Ferris DP, Czerniecki JM, Hannaford B. An ankle-foot orthosis powered by artificial pneumatic muscles. J Appl Biomech 2005;21:189-97.]. Nine young (23.3+/-1.6 years) and seven older (74.6+/-6.6 years) adults participated. For the young adults, eight out of nine increased their preferred walking speed when push-off power was augmented (1.18+/-0.16 to 1.25+/-0.16m/s, p=0.03). A similar, but non-significant, trend in preferred walking speed was observed for the older adults. With augmented push-off power, MCOT for young adults decreased from 0.395+/-0.057 to 0.343+/-0.047 (p=0.008); indicating that the neuromuscular system was able to adapt to use external energy to reduce metabolic cost. Only three older adults were tested but MCOT values showed a similar trend. Augmenting PF power increases gait speed and reduces MCOT in young adults. Older adults may need a longer period to take advantage of additional push-off power.