Synthetic seismograms for the Samail ophiolite, computed using the full reflectivity technique, are used to test the hypothesis that this ophiolite complex can serve as a prototype for young oceanic crust. If so, then this ophiolite complex can be compared with the older (45 m.y.) Bay of Islands (BOI) ophiolite to develop an aging model for oceanic crust. Synthetics for this young emplacement age (5--8 m.y.) ophiolite are compared with ocean bottom hydrophone data from 0.5- and 4.5-m.y.-age crust obtained during the Rivera Ocean Seismic Experiment. Second-arrival phases from reflected refractions from one ophiolite velocity-depth model and data from 4.5-m.y.-age crust are in excellent agreement: this agreement suggests that the Samail ophiolite is a good working model for young oceanic crust. Earlier work has shown that the BOI ophiolite is a good prototype for mature oceanic crust. The BOI ophiolite data also suggest that alteration of the lower oceanic crust and upper mantle is limited. We use the petrologic and geochemical properties of the Samail and BOI ophiolites to develop a two-stage model for crustal aging that directly limits alteration of the lower crust and upper mantle materials.