Populations of cities are ageing. To facilitate this shift, over 1100 cities joined the Global Network for Age Friendly Cities and Communities of the World Health Organization, which follow a 5-year cycle of planning, implementation and evaluation. Outdoor spaces and buildings, transportation and social environments are domains of the holistic age-friendly agenda. To improve the quality of these domains, cities in The Netherlands, Poland and Romania want to be capable to plan and implement adequate age-friendly strategies and to involve the views and experiences of their citizens. Hence, there is a need for participatory approaches for older people and policymakers, which combine quantitative, sustainable and community-based elements. City&Co aims to co-create an innovative spatial tool for a community-based assessment of the age-friendliness of cities, that can be used in multiple countries. This geoportal supports cities in the 5-year cycle of age-friendliness using direct input from participating older citizens. The tool and input for decision making and implementation will be developed with the direct engagement of older people and city workers. The creation and maintenance of local ecosystems of relevant stakeholders further supports the sustainability of age-friendly strategies and implementation and provides capacity building at city- and neighbourhood level.