Some of the major future challenges in urban planning are related to population ageing. Although the improvement of longevity can certainly be regarded as a human development success, many issues emerge in parallel, including social, economic, and spatial aspects. The Covid-19 pandemic experience and social distancing measures implemented have highlighted the need for compact communities and neighborhoods and in this context, urban theories promoting locality and accessibility have gained significant momentum. This paper focuses especially on the 15-minute city concept and sets its core element, an isochrone of 15 minutes of walking under scrutiny, to highlight how ageing places urban planning theories under stress. It tests the assumption that time-oriented theories of accessibility which rely on population-wide conventions may overestimate the mobility capacity and walking speeds of older people, that may lead to fallacies in spatial analyses and urban planning practice, especially in ageing societies. The findings suggest that a 30% adjusted equivalent, such as a 20-minute convention for the 15-minute city, might be more appropriate for older age groups. This contributes to the broader discussion about proximity and the walkable city, regardless of whether it is based on a 5-, 10-, 15-, or 20-minute model.