Forest plantations are widespread globally. Young forest plantations (hereafter young forests) differ from natural old-growth forests (hereafter old forests) in above- and below-ground structures, shaping water and carbon cycling processes. While above-ground differences are well studied, below-ground hydrology and biogeochemical processes remain poorly understood. Here we asked: How do hydrological flow paths and dissolved carbon processes belowground differ between young and old forests? Using a process-based hydro-biogeochemical model (BioRT-HBV) constrained by streamflow and dissolved organic and inorganic carbon (DOC and DIC) data, we analyzed three pairs of young-old forests at the H.J. Andrews Experimental Forest, Oregon, USA. Detailed simulations for a 57-year-old plantation (WS01) and a naturally regenerated ∼500-year-old forest (WS02) showed that the young forest had lower streamflow and smaller deep groundwater contributions (20%) than the old forest (30%). DOC was mainly produced in shallow soil but diverged with depth: transformed into DIC in the young forest and further produced in the old forest, yielding contrasting export patterns of flushing (DOC increases with discharge) and dilution (DOC decreases with discharge). These differences likely stem from variations in subsurface structures, supported by deeper, denser roots in old forest. Extending the analysis to two additional pairs showed (a) higher DOC and DIC concentrations in all old forests; (b) consistent DIC dilution patterns but variable DOC patterns. Numerical experiments indicate that these diverse DOC behaviors result from interactions among forest age, geology, and hydrological connectivity, and other factors, highlighting the overlooked role of forest development in subsurface carbon cycling.