Analysis of partial cross sections of 101 human tibiae indicated that osteon remodeling in the outer cortex is affected by age. The frequency of resorption spaces remained constant throughout life suggesting no loss of osteoclast function with age. However, the frequency of both forming osteons which were structurally complete but not completely mineralized increased with age. This suggests that protein matrix synthesis by osteoblasts slows with age and that initial mineralization, possibly mediated by osteoblasts, and final mineralization, possibly mediated by osteocytes, becomes increasingly deficient with increasing age. The frequency of osteons which have dense (sclerotic) inner lamellae decreases with age. This supports a hypothesis that such lammellae are functional, perhaps representing a specialized, labile, mineral phase and that osteons having this feature become less frequent as part of the general degenerative changes associated with aging.