Blue Space and Healthy Aging: Effects on Older Adults’ Walking in 15-Minute Living Circles—Evidence from Tianjin Binhai New Area

As global population ageing accelerates and urban governance increasingly prioritizes livability and age-friendly services, the 15-minute living circles concept has emerged as a key strategy to support daily walking exercise, social participation, and healthy ageing. In waterfront cities, blue spaces function as important everyday settings that contribute to environmental quality, recreational opportunities, and ecosystem services for older adults. This study extends the conventional 5D built environment framework by explicitly integrating blue space elements and characterizes older adults’ walking behaviour using four indicators across two dimensions (temporal and preference-based). We applied XGBoost regression and multiscale geographically weighted regression (MGWR) to identify threshold effects and spatial heterogeneity of blue space elements on older adults’ walking, and used K-means clustering to delineate blue space advantage zones within living circles. The results show that blue space accessibility, street scale, and water body density exhibit significant nonlinear relationships with older adults’ walking. Blue space elements shape walking behavior differentially and with pronounced spatial variation: in some living circles they encourage longer, recreational walks, while in others they stimulate high-frequency, short-distance walking. These effects produce destination preferences and time period preferences. The study highlights the pivotal role of blue spaces in age-friendly living circles and, based on spatial synergies among blue space advantage zones and their components, proposes renewal strategies including expanding the functional reach of blue spaces, constructing blue slow-walking corridors, and integrating blue–green symbiotic networks.