Biometric data describing the geometry and spacings of emmetropic human eyes were combined with lens shape and placement within the globe to generate paraxial models of image formation as a function of age. Three different representations of the shape of the internal refractive index gradient of the lens were evaluated--a Gullstrand-type model consisting of cortical and nuclear regions with different refractive indices, a power series model, and a linear-gradient model. All three refractive models satisfy the requirements for focus for all the data sets, indicating that lenticular refractive index gradient shape is essentially underdetermined in the paraxial limit. Lens refractive power decreases by almost 2 D during a 50-year period, and the concomitant decrease in system refractive power is due almost entirely to this effect. The reduction in spacing between the lens principal planes is a function of this, as is their anterior movement with age, and suggests that the compensatory processes maintaining far focus at the expense of near are not exactly balanced. Despite these changes in the lens contribution and their effect on the location of the system principal planes, which also move anteriorly, the spacing between the system principal planes remains constant. However, the trend toward reduced overall system power with age indicates the primary role of the lens in mediating image formation onto the retina over time.