This paper presents experimental and numerical studies of erythrocyte stretching, with a focus on the aging of red blood cells in an in vitro environment during storage. The experimental studies were performed using optical tweezers. The laser beam was used to pull and stretch a cell sedimented on a flat surface. A force calibration was obtained via a comparison of the experimental data with results from finite element simulations of the cell stretching. The experiments were performed using blood samples from blood bank donations made by three donors. The experiments were performed over 21 days of storage, and the estimate erythrocyte membrane shear modulus during this period increased from 2.5 to 13 mu N/m.