As yet the life sciences have provided no way to definitively, robustly measure biological age in an individual. In part this stems from a lack of consensus as to a useful definition of biological age, or indeed of aging more broadly. Researchers have long agreed upon sensible definitions at the high level, such as that aging is an increase with time in the risk of mortality due to intrinsic causes. That definition is validated, measurable over populations, but helps little when it comes to assessing the mortality risk or age of any given individual. At the low level, there are many specific forms of damage and dysfunction that can be measured, albeit not always without invasive sampling. Burden of senescent cells, loss of mitochondrial function, reduced average telomere length, slowed pace of cell replication, reduced grip strength, changes in a thousand biomarkers relating to immune function, and so forth. We have the general sense of trends, but again one cannot use these measures to say definite things about biological age and mortality risk for any given individual.
We live in a world in which measurements and algorithmic combinations of measurements that reflect aging in populations are proliferating alongside the interest in treating aging as a medical condition. This is particularly true for the aging clocks, such as epigenetic clocks, derived from machine learning techniques applied to large bodies of biological data. A slow, incremental ongoing process is underway to find out whether this landscape forms a suitable foundation for the discovery and development of a true consensus measure of biological age that can be applied usefully to individuals. At present that largely involves assessing as many people as possible using as many different measurement approaches as possible, and searching for patterns in the data. Data informs the way in which researchers think about definitions of aging, which inspires new approaches to measurement of biological age, and use of those approaches produces new data. It is a circular road.
