The study investigates the stochastic component of epigenetic aging by quantifying how much of the predictive accuracy of epigenetic clocks can be attributed to a stochastic process of DNA methylation (DNAm) change. Using DNAm data from sorted immune cells and whole-blood samples from 25 independent cohorts, the researchers built realistic simulation models to demonstrate that approximately 66–75% of the accuracy of Horvath’s clock and 90% of Zhang’s clock could be driven by a stochastic process. However, this fraction is lower (63%) for the PhenoAge clock, suggesting that biological aging is reflected by nonstochastic processes. The study further confirms that age acceleration in males, severe COVID-19 cases, and smokers is not driven by an increased rate of stochastic change but by nonstochastic processes. These findings deepen our understanding of the stochastic nature of epigenetic clocks and highlight the importance of nonstochastic mechanisms in biological aging.The study investigates the stochastic component of epigenetic aging by quantifying how much of the predictive accuracy of epigenetic clocks can be attributed to a stochastic process of DNA methylation (DNAm) change. Using DNAm data from sorted immune cells and whole-blood samples from 25 independent cohorts, the researchers built realistic simulation models to demonstrate that approximately 66–75% of the accuracy of Horvath’s clock and 90% of Zhang’s clock could be driven by a stochastic process. However, this fraction is lower (63%) for the PhenoAge clock, suggesting that biological aging is reflected by nonstochastic processes. The study further confirms that age acceleration in males, severe COVID-19 cases, and smokers is not driven by an increased rate of stochastic change but by nonstochastic processes. These findings deepen our understanding of the stochastic nature of epigenetic clocks and highlight the importance of nonstochastic mechanisms in biological aging.