While population ageing poses fiscal and dependency challenges for Thailand, conventional measures of population ageing may misrepresent future burdens by overlooking key societal changes under way. Given improvements in health and life expectancy which should slow down the pace of population ageing, this paper argues that the threshold of ‘old age’ (65+) should be redefined. Additionally, the analysis considers changes in labour force participation and productivity brought about by improved educational attainment and a shift from the informal to the formal sector. By using a microsimulation population projection model, the study forecasts the population of Thailand from 2020 to 2070 and calculates dependency ratios that incorporate these dimensions. The analysis highlights the mitigating potential of measures to facilitate economic formalization and human capital development.