Quantifying biological aging and predicting age-related functional decline are increasingly important for reducing the health and socioeconomic burden of population aging. DNA methylation is one of the most extensively studied epigenetic features of aging and underlies epigenetic clocks, promising quantitative biomarkers of biological age. However, the molecular processes driving age-associated DNA methylation changes and the biological signals captured by these clocks remain unclear. A major unresolved issue is the contribution of oxidized 5-methylcytosine (5mC) derivatives to epigenetic remodeling during aging. These derivatives, including 5-hydroxymethylcytosine (5hmC), 5-formylcytosine and 5-carboxylcytosine, are generated through TET-mediated oxidation of 5mC. Although they can act as intermediates of DNA demethylation, they may also represent epigenetic signals with regulatory functions distinct from 5mC. This review summarizes current evidence on oxidized 5mC derivatives in mammalian aging, focusing on 5hmC. Aging is associated with context-dependent changes in global 5hmC levels and genome-wide distribution. Age-associated differentially hydroxymethylated regions occur in genes, regulatory elements, repetitive sequences and transposable elements. 5hmC and 5mC changes often show distinct genomic distributions, suggesting partially independent epigenetic processes. 5hmC may regulate transcriptional stability, cellular identity, inflammation and tissue vulnerability. Understanding the TET/5hmC pathway may improve interpretation of epigenetic aging and support 5hmC-based clocks and healthy-aging strategies.
