Interior Douglas-fir (Pseudotsuga menziesii var. glauca {Beissn.} Franco), ponderosa pine (Pinus ponderosa Dougl. ex Laws), and western larch (Larix occidentalis Nutt.) are common associates from the southern interior of British Columbia into the United States. We developed years-to-stump-height and years-to-breast-height models that use site index to predict how long it takes a site tree (a tree whose height growth is indicative of site productivity) to reach stump height (0.3 m) and breast height (1.3 m). These models are useful for planning silviculture treatments, rotation schedules, and determining current and past stand structure. Nonlinear regression was used to develop the years-to-stump-height and years-to-breast-height models from stem analysis data. The fitted models show that years-to-stump-height and years-to-breast-height decline as site index increases. There was more variability in years-to-stump-height and years-to-breast-height over a range of site indices for ponderosa pine than for western larch and Douglas-fir. Equations to predict years-to-breast-height from years-from-stump-to-breast-height were derived from our models. These equations were linear and allow years-to-breast-height to be estimated from increment cores taken at stump height and breast height. The models developed in this study allow for better estimates of total tree and stand age for forest management planning and stand reconstruction.