With the progress of ecological restoration projects, the area of planted forests on the Loess Plateau is continuously increasing, but the response of planted forest ecosystem service function is still controversial. Based on the spatio-temporal distribution and planting year of planted forests on the Loess Plateau, this study assessed the dynamic changes in carbon sequestration and hydrological regulation service functions of planted forest ecosystems from three dimensions: net primary productivity (NPP), evapotranspiration (ET), and soil moisture content (SMC), examined the impacts of planted forests on natural forests and the overall regional ecosystem functions, and furthermore, investigated the influence of planted forest age on ecosystem service functions from 2000 to 2020. The proportion of NPP from natural forests relative to the entire region remained relatively constant, at around 21%. In contrast, the contribution of NPP from planted forests had shown a significant increase, surpassing that of natural forests in 2009 to become the primary driver of NPP growth on the Loess Plateau. The share of planted forests in the region's ET and SMC had also seen a notable rise, increasing from under 2% to 13.9% and from 1.5% to 11.8%, respectively. By 2015, the ET and SMC of planted forests on the Loess Plateau had gradually stabilized. As the age of the forest increased, the rates and quantities of SMC changes decreased. The positive correlation between ET and forest age and the negative correlation between SMC and forest age indicated that establishing planted forests of different age classes would effectively mitigate the impact of tree growth on the reduced soil moisture. While a significant correlation existed between the age of planted forests and the rate and magnitude of NPP, ET, and SMC changes (P < 0.01), the persistence of this relationship in the future, given the current predominance of young and middle-aged planted forests in the region, requires further investigation.