There are two things: - DNA Methylation - DNA Acetylation Methylation downregulates a gene's expression. Acetylation upregulates a gene's expression. In any c

There are two things: - DNA Methylation - DNA Acetylation Methylation downregulates a gene's expression. Acetylation upregulates a gene's expression. In any case, over your lifetime methylation profiles of your genetics can vary, generally increasing. Methylation of your cells isn't something that happens in every tissue at the same rate. So you actually need to define a biological clock based on methylation over time for all the different tissues. Then you can also build clocks for 2 or more tissues, or the whole organism... maybe https://en.wikipedia.org/wiki/Epigenetic_clock https://en.wikipedia.org/wiki/Epigenetic_clock