The impact energy absorption of human femoral cortical bone decreases by a factor of about three between the ages of three and ninety. This decrease is associated with, and partially caused by, an increased mineralization of the bone. The porosity of the bone, which is highest at each end of the life span, is not dangerous in young bone but seems to exacerbate the effect of high mineralization in the old bone. Increased mineralization probably acts by decreasing the plastic deformation undergone before fracture starts. It may also make the process of crack travel less energetically expensive.