Background: To our knowledge, there are few longitudinal studies of Vitamin D status from childhood to early adulthood, and it is uncertain whether Vitamin D predicts peak bone mass in young adults. Objectives: The purpose of this longitudinal study was to evaluate the long-term stability of Vitamin D status from ages ± to 20 y in healthy individuals and to study associations between serum 25-hydroxyVitamin D [25(OH)D] at different developmental stages and bone mass measured at age 20 y. Design: Participants were offspring of the Western Australian Pregnancy Cohort (Raine) study. Serum 25(OH)D was assessed at ages 6, 14, 17, and 20 y, and whole-body bone mineral content (BMC) and bone mineral density (BMD) were measured at age 20 y through the use of dual-energy X-ray absorptiometry (DXA). Our analysis included 821 participants (385 females) who had≥3 serum 25(OH)D measures and DXA data. We used latent class growth analysis and identified 4 Vitamin D status trajectories: consistently lower (n = 259), decreasing (n = 125), increasing (n = 138), and consistently higher (n = 299). Results: There were significant correlations between serum 25(OH)D concentrations at different time points in both sexes (r = 0.346-0.560, P <0.001), with stronger correlations at adjacent time points. In males, but not in females, serum 25(OH)D at ages 6, 17, and 20 y was positively associated with total-body BMC and BMD at age 20 y [covariate-adjusted increments of 40.7-53.9 g and 14.7-18.6 mg/cm-, respectively, per 25 nmol/L 25(OH)D]; when 25(OH)D at all 4 ages was included in the same model, the concentration at age ± y remained significant. Males in the "consistently higher" trajectory had 3.2-3.4% higher total body BMC and BMD than those who were in the "consistently lower" trajectory, accounting for age and anthropometric and lifestyle factors. Conclusions: Within both sexes, there are moderate associations between Vitamin D status measured in prepuberty, adolescence, and early adulthood. Vitamin D status in childhood is a significant predictor of peak bone mass in male but not female subjects. Am J Clin Nutr 2017;106:276-83.