An accurate projection for ambulance demand is essential to enable better resource planning for the future that strives to either maintain current levels of services or reconsider future standards and expectations. More than 2 million cases of emergency room attendance in 2008 were obtained from the Hong Kong Hospital Authority to project the demand for its ambulance services in 2036. The projection of ambulance demand in 2036 was computed in consideration of changes in the age-gender structure between 2008 and 2036. The quadratic relation between average daily temperature and daily ambulance demand in 2036 was further explored by including and excluding age-gender demographic changes. Without accounting for changes in the age-gender structure, the 2036 ambulance demand for age groups of 65 and above were consistently underestimated (by 38%-65%), whereas those of younger age groups were overestimated (by 6%-37%). Moreover, changes in the 2008 to 2036 age-gender structure also shift upward and emphasize relationships between average daily temperature and daily ambulance demand at both ends of the quadratic U-shaped curve. Our study reveals a potential societal implication of ageing population on the demand for ambulance services.