These authors used in situ small-angle X-ray scattering, wide-angle X-ray diffraction, synchrotron X-ray computed tomography and in situ fracture-toughness measurements in the scanning electron microscope to characterize effects the effects of aging on bone. They find that the loss in toughness with aging is due to increased nonenzymatic collagen cross-linking, which suppresses plasticity at nanoscale dimensions, and to an increased osteonal density, which limits the potency of crack-bridging mechanisms at micrometer scales.
http://www.ncbi.nlm.nih.gov/pubmed/21873221
Zimmermann EA, Schaible E, Bale H, Barth HD, Tang SY, Reichert P, et al Proc Natl Acad Sci U S A.