Stegosaurus has unique plate- and spike-shaped osteoderms. Previous studies have focused on the function of the osteoderms; however, ontogenetic development and maturity of these osteoderms with respect to Stegosaurus body growth are little known. In this study, the bone growth of both the skeleton and osteoderms was assessed using thin sections from small to large-sized individuals. In the small individual, the bone histology shows a fibro-lamellar tissue with a radial and/or reticular vascular network in both a body element and an osteoderm. In the medium-sized individual, the cortex shows a longitudinal vascular network and a few lines of arrested growth (LAG) in the skeleton, whereas the osteoderms still retain fibro-lamellar bone and show a reticular vascular network without LAGs. In large individuals, both body elements and osteoderms have fibro-lamellar tissue and multiple LAGs. However, bone tissues from body elements possess external fundamental systems (EFS) at their periphery, whereas osteoderms from the large individual lack EFS. Four histological stages are observed in both the skeleton and the osteoderms in the growth series of Stegosaurus sections: (1) a fibro-lamellar tissue with a radial and/or reticular vascular network, (2) fibro-lamellar tissue with a longitudinal vascular network, (3) with LAGs, and (4) with EFS. The timing of histological changes of osteoderms was delayed from those of the skeleton. This delay indicates that osteoderms might have maintained faster growth rates than the body elements after the maturity of the skeleton.