In summary, the basic idea of the 15MC states that cities should be designed or redesigned so that residents of all ages and social backgrounds have access to their everyday needs (housing, work, food, health, education, culture, and recreation) within an easily reachable distance set at 15 minutes. To accomplish this goal, the theory advocates a shift from private vehicles to active mobility (mainly walking and cycling), amid high residential densities, and through the mass adoption of digital technologies such as remote work or shared travel, which reduce unnecessary mobility and waste of time, in general \cite{c40,Moreno_2021,Allam_2021,Pozoukidou_2021}.

Although 15MC may initially be viewed as a rebranded proposition toward proximity, locality, mixed uses, and compact neighborhood design, it is often considered more literally as a planning practice or spatial analysis tool \cite{Abdelfattah_2022,Caselli_2022,Ferrer_Ortiz_2022,Di_Marino_2022,Gaglione_2021,Graells_Garrido_2021}. Then, 15MC refers then to an isochrone that expresses a radius that defines an area considered to be local. Isochrones are not new and have long been used in transport planning, although they have gained wider attention in recent years, mainly because of the use of digital GPS tracking applications that enable near real-time data flows and estimations for both users and researchers or the planning community. Isochrones are primarily constructed using two variables: time and speed. Average speed conventions regarding walking (or cycling or driving) are used to determine the radius that defines 'nearby' and as such, a question arises as to what extent such conventions concern most people within a city, and most specifically older ones, as will be further discussed in this paper.