The incidence of bone fracture increases with age, due to both declining bone quantity and quality. Towards the goal of an improved understanding of the causes of the age-related decline in the fracture toughness of male cortical bone, nanoindentation experiments were performed on femoral diaphysis specimens from men aged 21-98 years and tibiae from male Fischer F344 rats aged 6, 12, and 24 months (n = 6-11 per age group). Bone is an anisotropic viscoelastic material with a complex hierarchical structure, in which water plays a critical mechanical role by conferring ductility. Because aged bone has been found to have less matrix-bound water and dry bone is less viscoelastic, the nanoindentation method used was designed to be sensitive to changes in viscoelasticity. In both experiments, the specimens were tested dry in air and immersed in phosphate buffered saline solution. In the human femur experiment, longitudinal (n = 26) and transverse (n = 25) specimens were tested. The results for the human femur and rat tibia experiments were similar: indentation stiffness (storage modulus) and hardness were found to increase with age, while viscoelasticity (loss modulus) was unchanged. Further analyses used fracture, compositional (mineralization, bound water, crosslinking concentrations), and structural (porosity, osteonal area fraction) data collected by collaborators. The increases in indentation stiffness and hardness with age were best explained by increased mineralization with age. In both experiments, indentation stiffness and hardness were negatively correlated with the fracture toughness parameters, which is consistent with a tradeoff between material strength and toughness. In keeping with the complex structure of bone, the fracture toughness of the human femur specimens was best predicted by a combination of indentation, compositional, and structural terms. In conclusion, this study found increased stiffness and hardness with age in male cortical bone, a change which contributed to decreased fracture toughness. On the other hand, viscoelasticity was unchanged with age and was not associated with fracture toughness.