We tested the hypothesis that forest age influences the carbon isotope ratio (d13C) of carbon reservoirs and CO2 at local and regional levels. Carbon isotope ratios of ecosystem respiration (d13CR), soil respiration (d13CR-soil), bulk needle tissue (d13CP) and soil organic carbon (d13CSOC) were measured in > 450-, 40- and 20-year-old temperate, mixed coniferous forests in southern Washington, USA. Values of d13CR, d13CR-soil, d13CP and d13CSOC showed consistent enrichment with increasing stand age. Between the youngest and oldest forests there was a ~1‰ enrichment in d13CP (at similar canopy levels), d13CSOC (throughout the soil column), d13CR-soil (during the wet season) and d13CR (during the dry season). Mean values of d13CR were –25.9, –26.5 and –27.0‰ for the 450-, 40- and 20-year-old forests, respectively. Both d13CR-soil and the difference between d13CR and d13CR-soil were more 13C enriched in older forests than in young forest: d13CR – d13CR-soil = 2.3, 1.1 and 0.5‰ for the 450-, 40- and 20-year-old forests, respectively. Values of d13Cp were proportionally more depleted relative to d13CR: d13CR – d13CP = 0.5, 2.2 and 2.5‰ for the 450-, 40- and 20-year-old forests, respectively. Values of d13CP were most 13C-enriched at the top of the canopy and in the oldest forest regardless of season (overall values were –26.9, –28.7 and –29.4‰ for the 450-, 40- and 20-year-old forests, respectively). Values of d13CSOC from shallow soil depths were similar to d13CP values of upper- and mid-canopy needles. All d13C data are consistent with the hypothesis that a decrease in stomatal conductance associated with decreased hydraulic conductance leads to increased CO2 diffusional limitations in older coniferous trees. The strong associations between d13CP in needles with d13CR and d13CR-soil at the forest level suggest that 13C observations scale between leaf and ecosystem levels.