Demographic change brings complex challenges to the economic, social, and environmental systems of cities, with its asymmetric, overlapping, and compounded impacts across people and places. This paper analyses the main demographic trends and challenges in cities across OECD countries, with a focus on G7 cities. It disentangles trends and challenges across three distinct, yet interrelated, dynamics: i) ageing population and decreasing household size in cities (for instance, the number of older adults in cities increased by 2.4% per year in G7 countries between 2008 and 2018), ii) city population growth or decline (for instance, 20% of cities across OECD countries experienced population decline between 2001 and 2021), and iii) spatial segregation within cities (for instance, commuting zones of metropolitan areas are ageing faster than inner cities). Based on the evidence-based analyses, the paper provides policy insights to address the impacts of demographic change and to build more inclusive, sustainable, and resilient cities and urban systems, with city-specific examples. Preliminary findings from the paper served as input to the G7 Ministerial Meeting on Sustainable Urban Development (7-9 July 2023, Takamatsu, Japan) under the G7 Japanese Presidency.