Since 1980, the earnings share of older workers has risen in the United States, simultaneous with a historic decline in labor’s share of income. We hypothesize that an aging workforce has contributed to the decline in labor’s share. We formalize this hypothesis in an on-the-job search model, in which employers of older workers may have substantial monopsony power due to the decline in labor market dynamism that accompanies age. This manifests as a rising wedge between a worker’s earnings and marginal product over the life-cycle. We estimate the age profile of these wedges using cross-industry responses of labor shares to changes in the age-distribution of earnings. We find that a sixty year old worker receives half of her marginal product relative to when she was twenty, which, together with recent demographic trends, can account for 59% of the recent decline in the U.S. labor share. Industrial heterogeneity in this age profile is consistent with the monopsony-power mechanism: highly unionized industries exhibit no relationship between age and payroll shares.