Porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities. Porosity is particularly important in the uppermost young crust, where rapid variations in velocities with depth and crustal age are observed. Knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of Poisson's ratio are available from observations of compressional and shear seismic velocities vp and vs. In this paper I present a joint seismic waveform inversion for vp and vs; velocities are found while maximizing or minimizing Poisson's ratio using a hypothesis-testing mechanism. I apply this method to ocean bottom hydrophone data in 140 Ma Atlantic crust; the resulting solution corridor agrees with laboratory measurements without the low Poisson's ratio anomalies at depths of 0.8--1.5 km found by Spudich and Orcutt (1980) and Au and Clowes (1984) on younger (<15 Ma) Pacific crust. Compiling other published vp and vs solutions, an age-dependent pattern emerges: none of the solutions for crust older than 60 Ma display the Poisson's ratio anomaly. I propose a simple crustal evolution model, using thin and thick cracks, to explain these observations: thin cracks preferentially close at shallow depths in the crust, producing the localized Poisson's ratio anomaly. Sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements. This model is consistent with similar models of crack populations and their evolution from shallow measurements.