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Epigenetic “clocks” based on DNA methylation (DNAme) are the most robust and widely employed aging biomarker. They have been built for numerous species and reflect gold-standard interventions that extend lifespan. However, conventional methods for measuring epigenetic clocks are expensive and low-throughput. Here, we describe Tagmentation-based Indexing for Methylation Sequencing (TIME-Seq) for ultra-cheap and scalable targeted methylation sequencing of epigenetic clocks and other DNAme biomarkers. Using TIME-Seq, we built and validated inexpensive epigenetic clocks based on genomic and ribosomal DNAme in hundreds of mice and human samples. We also discover it is possible to accurately predict age from extremely low-cost shallow sequencing (eg, 10,000 reads) of TIME-Seq libraries using scAge, a probabilistic age-prediction algorithm originally applied to single cells. Together, these methods reduce the cost of DNAme biomarker analysis by more than two orders of magnitude, thereby expanding and democratizing their use in aging research, clinical trials, and disease diagnosis.