Understanding the impact of forest age transitions on the global net carbon balance is critical for advancing forest management and climate change mitigation strategies. We analysed changes in the global forest age (2010-2020) using the Global Age Mapping Integration (GAMI) v2.0 dataset alongside satellite-derived aboveground carbon (AGC) and atmospheric inversion-derived net CO2 flux data. We observe decreasing forest age in the Amazon, Congo Basin, and Southeast Asia, primarily in old-growth forests due to stand-replacing disturbances like clear-cutting followed by regrowth. Large patches of older Siberian forests, ranging from 80 to 200 years, transitioned to younger ages due to increased fire5 and harvest6. Despite stand-replacements, forests in China, Europe, and North America experienced a net ageing of nearly ten years on average. A substantial portion of the gradually ageing forests is located in South America Tropical (0.19 of total gradually ageing forest fraction, 0.64 billion hectares), Eurasia boreal (a fraction of 0.17, 0.56 billion hectares), Europe (a fraction of 0.10, 0.35 billion hectares), and North America temperate (a fraction of 0.094, 0.31 billion hectares). We find a significant correlation between stand-replaced forest fraction and the inversely derived 2010-2020 trend in carbon sink strength at global scales (R2 = 0.33, slope = +109.19 gC m-2 year-2, p-val < 0.001, N=60). This is partly due to the transition from carbon-rich old-growth forests (approximately 98.0 MgC ha⁻¹) to young stand-replaced forests (approximately 43.5 MgC ha⁻¹), resulting in a net AGC loss of +0.15 (+ denotes loss of AGC) PgC year⁻¹. When accounting for all stand-replaced forests, this loss increases to +0.43 PgC year⁻¹, representing approximately 1.6% of the total forest biomass (around 270 PgC in 2020) over ten years. Our findings highlight that shifts in forest age are crucial to understanding global carbon losses and gains. Understanding these dynamics is essential for developing forest management strategies that optimise harvesting methods and sequester more anthropogenic CO2.