Marine debris is a major stressor in the ocean environment, including in the California Current ecosystem. But it is almost impossible to tell how long this marine debris, especially small marine debris, has been in the ocean. We examined how plastic’s chemical structure changes with weathering, in order to compare experimentally weathered plastic to the weathering patterns of marine debris found in the ocean. Polypropylene, low-density polyethylene, and high-density polyethylene pre-production plastic pellets were weathered for three years in three treatments: dry/sunlight, seawater/sunlight, and seawater/darkness. Changes in chemical bond structures (hydroxyl, carbon double bonds, carbon-oxygen and carbon-nitrogen) with weathering were measured via Fourier Transform Infrared (FTIR) spectroscopy. These indices from experimentally weathered particles were compared to microplastics collected from oceanic surface waters in the California Current, the North Pacific Subtropical Gyre, and the transition region between the two. Although our indices had nonlinear changes with environmental exposure, they can potentially approximate the age of plastics. The majority of the oceanic polyethylene particles we measured have inferred ages >9 months, with some > 30 months, and the polypropylene samples are consistent with ages > 18 months.