Walking behavior may vary across age groups, highlighting the need for a validated tool to assess walking including older adults. While the Japanese version of the International Physical Activity Questionnaire long form (Japanese IPAQ-L) has been validated for total physical activity in younger populations, its validity in assessing walking behavior remains unclear in any age group. This study aimed to evaluate the concurrent validity of the Japanese IPAQ-L for assessing walking behavior throughout adulthood, with a special focus on its applicability to older adults. Participants wore an ActiGraph wGT3X-BT (ActiGraph LLC, Pensacola, Florida, United States), an accelerometer-based activity monitor, around their waist for seven consecutive days to objectively measure their physical activity. On the following day, they completed the Japanese IPAQ-L, which asked about their domain-specific physical activity over the preceding seven days. Spearman's rank correlation coefficients (ρ) were calculated to examine the associations of walking (total, work, transport, leisure) and moderate physical activity, both subjectively assessed by the Japanese IPAQ-L, with objectively measured moderate physical activity (3.00-5.99 metabolic equivalents) by the ActiGraph. All 130 recruited participants completed the data collection. Of these, 113 participants without missing data (aged 23-89 years; 51.3% women) were included in the analysis. Self-reported total walking showed weak to moderate correlations with objectively measured moderate physical activity across all age groups, with the strongest correlation observed in the oldest age group (ρ=0.32 for the total group; ρ=0.38 for the youngest age group; ρ=0.35 for the middle-aged group; ρ=0.55 for the oldest age group). For domain-specific walking, the correlations varied by age group. Meanwhile, no meaningful correlations were found for self-reported moderate physical activity, regardless of whether walking was included. The Japanese IPAQ-L showed weak to moderate concurrent validity for assessing total walking when compared to accelerometer-based activity data across age groups. Notably, the validity was higher in older adults. These findings suggest that the Japanese IPAQ-L can be used to rank individuals based on their total walking, with reasonable applicability to older adults.