After an initial period of less than one year, the relative decrease of creep rate with the age at loading is much stronger than the relative decrease of the growth rate of hydration degree. This suggests another source of long-term aging, which is explained by relaxation of the so-called microprestress. We conceive the microprestress as an overall characteristic of the disjoining pressures in nanopores filled by hindered adsorbed water and the counterbalancing tensile stresses in the nanostructure of hydrated cement. We model mathematically how the microprestress gets generated by the volume changes due to hydration as well as the pore water content changes and temperature changes. To anchor the model physically, we discuss the pore structure of hydrated cement and water adsorption on its enormous internal surface. We also present an alternative computational approach in which the microprestress changes are replaced by viscosity variation and show how the microprestress theory is easily incorporated into finite element programs for creep.