Forest age structure, shaped by past land-use and land-cover changes (LULCC), is pivotal for estimating ecosystem carbon sinks. China's extensive LULCC in recent decades has led to a complex forest age structure, but its impact on the carbon sink remains uncertain. Here, using a process-based ecosystem model with an explicit representation of forest age cohorts, we estimate China's terrestrial carbon sink as 198 ± 54 TgC yr⁻¹ in the 2010s, mainly contributed by middle-aged forests. The existing forests in 2020 contribute most to the future total carbon sink, but its contribution will decrease significantly by −1.1∼−0.35 TgC yr⁻¹ until 2100 due to forest aging and the slowdown of CO₂ concentration growth. Future re/afforestation will enhance carbon sink by increasing forest area and rejuvenating forest demography. Our study emphasizes the limited future carbon sink due to forest aging, implying that realizing China's carbon neutrality should not rely excessively on ecosystem carbon sink.