Viscoelasticity is a sensitive measure of the microstructural constitution of soft biological tissue and is increasingly used as a diagnostic marker, e.g. in staging liver fibrosis or characterizing breast tumors. In this study, multifrequency magnetic resonance elastography was used to investigate the in vivo viscoelasticity of healthy human brain in 55 volunteers (23 females) ranging in age from 18 to 88 years. The application of four vibration frequencies in an acoustic range from 25 to 62.5 Hz revealed for the first time how physiological aging changes the global viscosity and elasticity of the brain. Using the rheological springpot model, viscosity and elasticity are combined in a parameter mu that describes the solid-fluid behavior of the tissue and a parameter alpha related to the tissue's microstructure. It is shown that the healthy adult brain undergoes steady parenchymal 'liquefaction' characterized by a continuous decline in mu of 0.8% per year (P<0.001), whereas alpha remains unchanged. Furthermore, significant sex differences were found with female brains being on average 9% more solid-like than their male counterparts rendering women more than a decade 'younger' than men with respect to brain mechanics (P=0.016). These results set the background for using cerebral multifrequency elastography in diagnosing subtle neurodegenerative processes not detectable by other diagnostic methods.