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Despite the female predominance in age- and immune-associated diseases, the pathways underlying sex differences in healthy immune aging remain incompletely understood. Here, we analyze a multi-ethnic single-cell transcriptome of healthy human immune aging (35% Asian), comprising 3.8 million peripheral blood mononuclear cells (PBMCs) from 1,828 individuals aged 19-97 years. Prominent peaks of differential gene expression around 40 (mainly CD4 T cells) and after 60 (mainly CD8 T cells) years of age were accompanied by an age-dependent decline in RNA/protein homeostasis and inflammatory PBMC polarization with sex-dependent kinetics. While females displayed sustained CD8 T cell activation and late-life aging signatures in CD4 T, NK, and B cells, males exhibited early-life fluctuations in CD4 T cell immunometabolism associated with hypomethylation of SSH3 at chromosome 11q13. Deep learning-based biological age clocks stratified for sex outperformed sex-combined models, learning from transcriptional immune trajectories. We thus unravel a nonlinear PBMC aging whereby targeting sex-specific pathways might allow precision geromedicine.