DNAm PhenoAge is a group of biomarkers developed at Yale University that allows you to calculate the biological age of a person. It is based on data from NHANES III (a large health study from the US) and includes 9 biomarkers, including the number of red and white errors, as well as a person's chronological age. It is able to characterize the risks of different aging outcomes to a certain extent and provides insight into important aging pathways. The DNAm PhenoAge biomarker has been validated in NHANES IV as a predictor of morbidity and mortality risk, and has been tested to be warranted in multiple patient cohorts, taking into account age-related outcomes.
Biological age is according DNAm PhenoAge is a reasonably accurate predictor of health and works at the epigenetic level. For most people, this can be a good indicator of health status.