Contextualizing aging clocks and properly describing biological age

Abstract

Usage of the phrase "biological age" has picked up considerably since the advent of aging clocks and it has become commonplace to describe an aging clock's output as biological age. In contrast to this labeling, biological age is also often depicted as a more abstract concept that helps explain how individuals are aging internally, externally, and functionally. Given that the bulk of molecular aging is tissue-specific and aging itself is a remarkably complex, multifarious process, it is unsurprising that most surveyed scientists agree that aging cannot be quantified via a single metric. We share this sentiment and argue that, just like it would not be reasonable to assume that an individual with an ideal grip strength, VO₂ max, or any other aging biomarker is biologically young, we should be careful not to conflate an aging clock with whole-body biological aging. To address this, we recommend that researchers describe the output of an aging clock based on the type of input data used or the name of the clock itself. Epigenetic aging clocks produce epigenetic age, transcriptomic aging clocks produce transcriptomic age, and so forth. If a clock has a unique name, such as our recently developed epigenetic aging clock CheekAge, the name of the clock can double as the output. As a compromise solution, aging biomarkers can be described as indicators of biological age. We feel that these recommendations will help scientists and the public differentiate between aging biomarkers and the much more elusive concept of biological age.

Keywords: aging biomarker; aging clock; biohorology; biological age; chronological age; mortality.

FIGURE 1

Usage of the phrase “biological age” in PubMed over time. On September 12th, 2024, all PubMed results returned from the search “biological age” were downloaded as a CSV file. Using R version 4.3.3 and RStudio version 2024.04.2, the CSV file was converted to a data frame and visualized using the built‐in barplot() function. This figure was specifically made by plotting publication count on the y‐axis and publication year on the x‐axis.