Background: The neighbourhood built environment (BE) provides opportunities for regular walking for transport (WfT). Within the same city, age differences in WfT can vary significantly across neighbourhoods, although little is known about the reasons for this variation. This cross-sectional study investigated the contribution of the BE to explaining age differences in WfT across neighbourhoods. Methods: This investigation used baseline (2007) data from the How Areas in Brisbane Influence HealTh and AcTivity (HABITAT) Study. The sample included 11,035 residents aged 40–65 years living in 200 neighbourhoods in Brisbane, Australia (68.4% response rate). Self-reported weekly minutes of WfT were categorized into none (0 mins) and any (1–840 mins); age was categorized into 40–48, 49–57 and 58–65 years. Objectively assessed neighbourhood-level measures of the BE included residential density, street connectivity and land-use mix. Analyses involved multilevel binomial logistic regression with age as main predictor, adjusting for gender, socioeconomic position, residential self-selection, and neighbourhood disadvantage. Results: On average, older adults were significantly less likely to walk for transport. Age differences in WfT seemed to vary significantly across neighbourhoods, and the magnitude of the variation for older groups was twice that of their younger counterparts. The environmental measures analysed played a relatively limited role in explaining neighbourhood differences in the age-WfT relationship. Residential density and street connectivity explained up to 13% and 9% respectively of the observed between-neighbourhood variation in WfT across age groups. Conclusion: Neighbourhood-level factors semeed to influenced the WfT of younger and older adults differently, with older adults being more sensitive to their neighbourhood environment. In Brisbane, age differences in WfT were smaller in areas with higher residential density and street connectivity. These results favor the ongoing investigation of demographic heterogeneity around neighbourhood averages in other urban contexts to inform tailored ecological interventions that facilitate WfT for all age groups everywhere, supporting active aging communities.