This study evaluated the process by which core indicators of Age-friendly Cities were measured in diverse contexts worldwide. The indicators were developed through research and international expert consultation, and piloted in 15 communities from 12 countries in 2014-2015. Pilot sites attempted to measure all indicators, covering the physical and social environment, quality of life and equity, using locally available data. As a result, the majority of pilot sites reported data for each indicator using one of two possible operational definitions, though data availability varied. Often the indicators were measured using a modified or an alternative definition. Fidelity of measurement was generally higher for the physical environment indicators than for the social environment indicators. Assessing equity was seen as essential but challenging in practice. The core indicators provide an adaptable method for measuring the physical accessibility and social inclusiveness of communities and their impact on wellbeing and equity, especially for older adults. It provides a tool for closing the gap in locally derived data on age-friendliness and improving accountability. Selecting, measuring and reflecting on the indicators through an inclusive process helps to ensure the relevance and validity of the data and to raise awareness and promote collaboration. These, in turn, can become drivers of change for communities at any stage of progress in becoming age-friendly.