Despite its iconic status as the king of dinosaurs, Tyrannosaurus rex biology is incompletely understood. Here, we examine femur and tibia bone microstructure from two half-grown T. rex specimens, permitting the assessments of age, growth rate, and maturity necessary for investigating the early life history of this giant theropod. Osteohistology reveals these were immature individuals 13 to 15 years of age, exhibiting growth rates similar to extant birds and mammals, and that annual growth was dependent on resource abundance. Together, our results support the synonomization of “Nanotyrannus” into Tyrannosaurus and fail to support the hypothesized presence of a sympatric tyrannosaurid species of markedly smaller adult body size. Our independent data contribute to mounting evidence for a rapid shift in body size associated with ontogenetic niche partitioning late in T. rex ontogeny and suggest that this species singularly exploited mid- to large-sized theropod niches at the end of the Cretaceous.