Although the atomistic structure and the deformation characteristics of calcium silicate hydrates (C-S-H) are of primary interest in cement chemistry, they have not been fully investigated. In this study, pair distribution function (PDF) analysis was conducted on a 53-year-old fully hydrated tricalcium silicate (C₃S) paste using in situ synchrotron high-energy X-ray scattering to probe the atomic structural deformation of C-S-H under external loading. The results were compared with those from our previous PDF study of a 131-day-old C₃S paste in order to elucidate the effect of aging on the mechanical characteristics of C-S-H. Three different strains measured by the strain gauge, by the lattice shifts (d-spacing) in the reciprocal space, and by the shift of the interatomic distance (r) in the real space were compared. In the range of r < 20 Å, where most of the information was derived from C-S-H, the 53-year-old C₃S paste had a higher overall elastic modulus (18.3 GPa) and better resistance to compressive stress than the 131-day-old C₃S paste (elastic modulus: 8.3 GPa). Moreover, it was found that the macroscopic strains of the 53-year-old C₃S paste were presumably induced by mechanical deformation such as microcracks at the macroscale. The results provide experimental evidence for the atomistic and mesoscale mechanical behavior of C-S-H in the early and late ages.