Understanding varying climate responses in tree-ring data across tree ages is important, but little is known about tree-age effects on climate responses in tree-ring stable isotopes. To detect whether age differences in tree-ring delta C-13 and delta O-18 could lead to differing climate responses, we measured tree-ring cellulose delta C-13 and delta O-18 (1901-2010) from Schrenk spruce (Picea schrenkiana) trees in northwestern China with ages ranging from 110 to 470 years, which we binned into three age groups. Tree-ring delta C-13 (pin-corrected) and delta O-18 exhibited similar year-to-year variability between age groups and did not feature age-related trends. delta C-13 series from old trees (270-470 years) showed stronger legacy effects, reflecting influences from the antecedent period (due to carbohydrate reserves and climate), compared to young trees (110-125 years). Both tree-ring delta C-13 and delta O-18 values decreased with increasing relative humidity (RH) and precipitation and with decreasing mean and maximum temperatures during the main growing season (May-August). delta C-13 and delta O-18 exhibited age-dependent climate responses: Young trees had a stronger climate response in delta C-13 but a weaker or similar climate response in delta O-18 compared to old trees. We developed multiple growing-season RH reconstructions based on composite chronologies using delta C-13 and delta O-18 series from different age groups. In particular, we found that including delta C-13 from young trees improved the skill of RH reconstructions because of the age-specific mechanisms driving the delta C-13-climate relationship, but that caution is warranted with regard to extreme values. We therefore suggest that young trees should be considered when using stable isotopes, particularly in delta C-13, for climate reconstruction.