Aging is associated with complex molecular and cellular processes that are poorly understood. Here we leveraged the Tabula Muris Senis single-cell RNA-seq dataset35 to systematically characterize gene expression changes during aging across diverse cell types in mouse. We identified aging-dependent genes in 76 tissue-cell types from 23 tissues and characterized both shared and tissue-cell-specific aging behaviors. We found that the aging-related genes shared by multiple tissue-cell types change their expression congruently in the same direction during aging in most tissue-cell types, suggesting a coordinated global aging behavior at the organismal level. We integrated the aging-related genes to construct a cell-wise aging score that allowed us to investigate the aging status of different cell types from a transcriptomic perspective. Overall, our analysis provides one of the most comprehensive and systematic characterization of the molecular signatures of aging across diverse tissue-cell types in a mammalian system.