Two people can be the same age but show different RNA-module aging. What would a module clock show?

Answer: Which system aged faster

Exact birthday age — Birthday age is the simplest clock, but it is exactly what biology complicates. Two 70-year-olds can differ in inflammation, metabolism, muscle repair, or tissue structure. A transcriptomic clock estimates age-like molecular patterns from gene activity, not from a calendar. The more interesting twist is module-specific clocks: they can ask whether a particular pathway looks older than the rest of the body.

Whole body one score — A whole-body score is the simpler biological-age idea, but it is exactly what the module view refines. One overall number can hide whether inflammation, metabolism, tissue structure or another process is driving the signal. A module clock asks what biological process is carrying an aging-like signature right now. That makes it closer to a dashboard than a single grade.

Which system aged faster ✓ — Correct. A module clock can show whether one biological system is aging faster than another. The Nature paper describes pathway-specific effects: chronic diseases mainly accelerated inflammatory-module aging, while some lifespan-modulating interventions affected mitochondrial and metabolic modules. That is the payoff. "Biological age" becomes less like a single number and more like a set of needles moving at different speeds.