Objective metrics that change the most as we age

Summary

Objective metrics that change the most as we age Brandon Ballinger May 23, 2026 Yeats wrote of an aged man as “a tattered coat upon a stick.” But for most of us, aging comes as small diminishments: the morning that takes a little longer to shake off, the flight of stairs you notice, the cold that lingers a week instead of three days, the run that used to feel easy. We register these shifts as fatigue or a busy season before we register them as time. The body is keeping score long before we are. Each of these feelings has a number behind it. The morning that takes longer to shake off lines up with kidney and liver filtration slowing down. Two glasses of wine hit differently at forty than at twenty, and as eGFR drops a few points per decade, the cleanup takes longer. The flight of stairs you notice shows up as red blood cells slowly enlarging, each one a little less efficient at carrying oxygen. The cold that lingers a week instead of three days is your lymphocyte counts thinning, the frontline cells that recognize and clear viruses. The run that used to feel easy tracks with hemoglobin A1c creeping upward, insulin sensitivity stiffening and muscles slower to pull glucose from the blood. None of these metrics move more than a few percent in a year, but stacked across decades, they describe an aging body. Which biomarkers rise and fall fastest with age? Which of these can be stopped, slowed, or reversed? In this post we’ll go through them. The biomarkers that rise and fall the most with age. eGFR is the standard measure of how fast your kidneys clear waste from the blood, and it’s the runaway leader on this list. In our data it falls roughly 6 to 7 points per decade after age 20, from a typical 110 mL/min/1.73m² in the twenties to around 75 by the eighties. Its correlation with age (r = -0.51) is nearly twice as strong as the next-best signal on the panel. The drop matches the consensus from the Baltimore Longitudinal Study of Aging, which has tracked the same loss for...