Brain aging represents a critical risk factor for neurodegenerative diseases and cognitive decline, yet the measurement of biological brain age remains challenging. Brain aging clocks, which quantify the discrepancy between predicted brain age and chronological age, have emerged as powerful tools for assessing brain health and predicting disease outcomes. Recent advances have transformed these clocks from simple global metrics to sophisticated, multi-modal approaches that capture regional heterogeneity, measure the pace of aging, and achieve cellular resolution. This review examines the methodological evolution of brain aging clocks, including the development of regional brain age gradients, pace-of-aging measurements, and multi-modal integration strategies. We then explore the cellular and molecular mechanisms underlying accelerated brain aging, with particular emphasis on cellular senescence, cell-type-specific aging patterns, vascular dysfunction and blood-brain barrier breakdown, mitochondrial decline, proteostasis failure, synaptic loss, and the accumulation of senescent cells in neurodegenerative conditions. Epigenetic clocks and emerging plasma biomarkers (neurofilament light, GFAP, phosphorylated tau), particularly DNA methylation-based approaches, are discussed in the context of their relationship with neuroimaging markers and cognitive outcomes. Clinical applications are reviewed, including the prediction of neurodegenerative disease, the impact of socioeconomic and geographic disparities on brain aging, and emerging senotherapeutic interventions. Finally, we address current challenges in biomarker standardization, the need for longitudinal validation, and future directions toward precision aging medicine. Together, these advances position brain aging clocks as essential tools for understanding neural aging mechanisms and developing targeted interventions to promote healthy brain aging. SIGNIFICANCE STATEMENT: As populations age globally, predicting who will develop dementia or cognitive decline before symptoms appear has become a critical medical challenge. Brain aging clocks - tools that measure whether a person's brain appears biologically older or younger than their chronological age - offer a promising solution. This review explains how these tools have advanced from simple brain scans to sophisticated methods that detect aging at the level of individual cell types, and how "zombie cells" called senescent cells drive accelerated brain aging. We also show that brain aging may be slowed through lifestyle changes and emerging drugs, though robust human efficacy trials are ongoing. These insights open new paths toward earlier diagnosis and personalized treatments for Alzheimer's disease and other brain disorders.
