<p>Travel distance is a key indicator of human activity, reflecting individuals’ access to urban resources and their interactions with the built environment. Understanding its variation across demographic groups is particularly important in mountainous cities, where terrain constrains mobility and shapes spatial development. However, limited attention has been paid to how built environment, socioeconomic and topographic conditions jointly influence travel behaviour across age groups in rapidly urbanising contexts. This study addresses this gap by integrating mobile phone signaling data with a nonlinear modelling framework to analyse travel distance across different age groups in Guiyang, a representative mountainous city in China. The results show significant age-based differences in travel distance, peak travel times and the effects of built environment, socioeconomic, and topographic factors. The effective service ranges of major activity centres vary across age groups, and several factors, including nighttime light intensity, distance to the central business district, and employment density, exhibit pronounced nonlinear and threshold effects on travel distance. Younger and middle-aged groups are more strongly associated with employment accessibility, whereas older adults are more closely related to population density. These findings contribute to a better understanding of age-differentiated travel behaviour in mountainous urban contexts and provide empirical support for more age-friendly urban planning and transport policies.</p>