Biogenic volatile organic compound (BVOC) emission factors (E_s) underpin air quality and climate models, yet current databases intermingle data from both seedlings and mature trees and from two enclosure techniques, leaf cuvettes and dynamic branch chambers, whose comparability has rarely been rigorously tested. Here we quantified BVOC emissions from <em>Eucalyptus urophylla</em> by pairing the two methods on a statistically representative number of 2-month-old seedlings in the laboratory and 2-year-old trees measured at a managed plantations in subtropical China. Leaf-cuvette and branch-chamber determination of isoprene E_s matched within 5 % for both age classes, demonstrating method equivalence. In contrast, tree age exerted a significant impact on both the magnitude and speciation of emissions. Seedlings emitted ~50 % more isoprene and were enriched in cyclic monoterpenes like &alpha;-pinene and 1,8-cineole, whereas field-grown trees shifted toward highly reactive acyclic monoterpenes, with &beta;-ocimenes accounted for over 85 % of the terpene flux. These ontogenetic shifts imply that one-third of the entries in global E_s compilations, which are derived from seedling studies, likely overestimate local isoprene fluxes while under-representing the atmospheric reactivity of mature canopies. Our results validate the use of either chamber type for measuring isoprene E_s, highlight the need for improved analytical sensitivity before extending this equivalence to terpenes, and call for systematic, large-sample, branch-level measurements of adult trees to produce representative E_s values. Incorporating age-resolved emission factors into models will refine estimates of ozone and secondary organic aerosol formation in fast-growing subtropical plantations and other managed forests worldwide.