Biological age estimates how “old” your body is in physiological and molecular terms, which can differ from chronological age. Multiple methods target different aging hallmarks—DNA methylation, telomeres, gene expression, proteins, metabolites, functional tests, and the gut microbiome—each with strengths and limitations. For an in-depth resource focused on gut-based approaches, see [Overview of Biological Age Measurement Methods](https://www.innerbuddies.com/blogs/gut-health/measuring-biological-age-epigenetic-microbiome-methods). Epigenetic clocks (DNA methylation clocks) analyze methylation at specific genomic sites correlated with age. Examples include the Horvath and Hannum clocks, and newer models such as PhenoAge and GrimAge. Strengths: high accuracy, reproducibility, and strong associations with mortality and disease risk. Limitations: require DNA samples and specialized assays, and can be costly. Epigenetic measures are widely considered the current gold standard for molecular biological age estimation. Telomere length measures the protective caps at chromosome ends that shorten with cell division and stress. It’s a familiar biomarker and relatively easy to assay from blood cells. However, telomere length shows high inter-individual variability and a weaker correlation with clinical outcomes compared with methylation-based clocks. Transcriptomic clocks use gene expression patterns to infer age-related changes. They provide insight into active biological processes and can be tissue-specific, which is both a strength and a complication. Expression profiles are sensitive to environmental conditions and acute states, so standardization remains a challenge. Proteomic and metabolomic clocks examine proteins and small metabolites, respectively. Proteomic approaches can detect functional and disease-related signatures, while metabolomic profiles reflect metabolic and cellular health. Both fields are promising but technically complex and influenced by current health status, diet, and sampling conditions; they are still evolving as standardized aging measures. Physiological and functional biomarkers aggregate clinical measures such as grip strength, blood pressure, lung function, and inflammatory markers. These are clinically accessible and capture whole-body function, but lack the molecular precision of genomic or multi-omic clocks and can be affected by acute illness or lifestyle factors. Gut microbiome–based biological age leverages the composition and diversity of gut bacteria to estimate age. The microbiome shifts in predictable ways with aging: some species decline, others increase, and these changes relate to immune and metabolic health. Advantages include non-invasive stool sampling and the ability to reflect host–microbe interactions relevant to systemic aging. Limitations: the field is newer, microbiome profiles are highly influenced by diet, antibiotics, geography, and lifestyle, and models are less standardized. Accuracy is emerging—moderate today and improving with larger datasets and enhanced algorithms. For readers interested in practical testing considerations and how consumer and healthcare approaches differ, see Gut microbiome testing: why different for consumers and healthcare professionals. For applications in athletic performance and recovery, consult Gut microbiome & endurance sports. A testing option reference is available at microbiome test product page. Combining complementary methods (e.g., epigenetic clocks plus microbiome and physiological measures) tends to give a more holistic estimate of biological age. As multimodal datasets and modeling improve, integrated approaches will likely provide the most robust and actionable assessments.