In Iran’s population policy, fertility enhancement is often regarded as equivalent to population rejuvenation and the primary strategy for counteracting population ageing. However, increases in births, changes in population size, and changes in age structure are distinct demographic processes with different time horizons. Using a quantitative-analytical approach based on the United Nations World Population Prospects (WPP 2024), this study compares low, medium, and high fertility scenarios for Iran from 2025 to 2065. Demographic momentum and the Guillot relationship within stable population theory are utilized to theoretically interpret the structural patterns. Findings indicate that while fertility differences lead to substantial long-term divergence in total population size (reaching a 26.7-million-person gap between high and low scenarios by 2065), the age-structural response is highly asymmetric. Although the child share is sensitive to fertility and the elderly share rises across all scenarios, the working-age population share (15-64) remains remarkably stable near 56%. This structural inertia demonstrates that demographic momentum and generational lags preclude immediate population rejuvenation. Consequently, higher fertility alone cannot reverse population ageing, necessitating comprehensive policy planning adapted to ageing realities and focused on labor productivity.