<p>Aging arises not only from intrinsic cellular decline but also from systemic alterations in circulating factors that govern tissue maintenance and regeneration. Recent multi-omics advances — including plasma proteomics, metabolomics, and single-cell immunomics — highlight blood as both a mirror and a modulator of organismal aging. Circulating proteins and metabolites reflect not only chronological and biological age but also organ-specific aging trajectories, serving as robust predictors of healthspan, longevity, and disease risk. Beyond their diagnostic value, blood-borne components actively dictate the tempo of aging by shaping immune remodeling, metabolic homeostasis, and interorgan communication. Youthful circulation, defined as the blood-borne systemic environment of young individuals, promotes tissue homeostasis and regeneration and, when experimentally transferred via heterochronic parabiosis or young plasma transfer, induces transcriptomic, metabolic, and epigenetic rejuvenation across multiple tissues. Specific fractions — such as small extracellular vesicles, plasma proteins, and metabolites — restore mitochondrial function, suppress inflammation, and extend lifespan in animal models. Conversely, reducing pro-aging factors through plasma dilution or therapeutic plasma exchange mitigates age-associated decline and shows translational promise in neurodegenerative disease. Collectively, these insights position blood as a central regulatory axis of aging. In this Review, we synthesize current mechanistic and translational evidence on blood-borne aging regulators to outline a molecular framework for rejuvenation biology and future therapeutic development.</p>