Background Deficits in cognitive domains, in particular, those related to the prefrontal cortex, contribute to diminished walking performance in complex conditions in older age. Studies using functional near infra-red spectroscopy (fNIRS) reported inconsistent findings of brain activation age-related changes in response to increased task demands. We aimed to study the effects of aging on gait and prefrontal activation in complex walking tasks with internal and external task demands. Methods Twenty-three healthy young adults (30.9 ± 3.7 yrs) and 20 healthy older adults (69.7 ± 5.8 yrs) participated in this study. Gait and prefrontal activation were assessed during three walking conditions: (1) usual walking, (2) dual tasking (internal task demands) and, (3) obstacle negotiation (external task demands). fNIRS measured changes in oxygenated hemoglobin concentrations in the prefrontal cortex. Results Several gait measures were worse in older compared to younger adults under all walking conditions (p < 0.005). Even at the lowest level of challenge, older adults had significant increases in HbO2 levels during usual walking, relative to standing (p = 0.006). Both groups showed increased activation during dual-task (p < 0.002) and during obstacle negotiation (p < 0.003). Conclusions Prefrontal activation during walking is dependent on age and task properties and that older adults apparently rely more on cognitive resources even during usual walking task.