Not everything about you is the same age.

One birthday, several ages

You have one date of birth. It's a fact, and it's useful for filling in forms.

It is also, as a description of you, remarkably blunt. Two people born on the same day can have very different hearts, very different blood vessels and very different ways of handling a meal — and the gap between them isn't small.

This used to be an intuition. In the last few years it has become something closer to a measurement.

What the research actually found

In 2023 a team at Stanford published work in Nature showing that proteins circulating in blood carry distinct signatures for different organs — and that those signatures age at different rates within the same person. Someone can have a heart that looks older than their years and a set of kidneys that looks younger, at the same time, on the same day.

A second 2023 paper, in Nature Medicine, found the same heterogeneity across multiple organ systems using a different method entirely, and that these differences carried information about long-term health that a single overall number did not.

Two independent groups, two different measurement approaches, the same conclusion: ageing isn't one process happening at one speed. It's several, and they can diverge.

“How old is my body” is not very actionable. “Which part of this is moving fastest, and is it a part I can do anything about” is a question you can act on.

So what's worth tracking?

If you're the kind of person who already owns a wearable, you probably have more numbers than you know what to do with — and a nagging sense that most of them aren't telling you much. That instinct is broadly right. Here's a way to think about which ones earn their place.

• Things that are well-evidenced and easy to track. Cardiorespiratory fitness is the strongest single one — the association with long-term mortality is large, consistent, and about as close to settled as this field gets. Sleep regularity is in the same tier. Both are tracked well by almost any decent wearable, and if you’re only going to watch two things, watch these.
• Things that are well-evidenced and hard to track. Blood pressure across the whole day is the clearest example. Ambulatory pressure — measured repeatedly, over a full day and night, rather than once in a clinic — predicts outcomes better than an office reading does. Almost nobody does it, because until recently the only way was to wear an inflating cuff for twenty-four hours.
• Things that matter and get skipped. How your blood sugar behaves is the obvious one. Not whether it's high — most people asking this question are perfectly healthy — but how much it swings, and how quickly it settles. That's a picture that has historically required a sensor in your arm, so most people never see it at all.

The pattern here is worth noticing. The two signals with the strongest evidence and the worst coverage are blood pressure over the whole day, and glucose over the whole day. They're not skipped because they don't matter. They're skipped because measuring them has been genuinely inconvenient.

The honest bit

A wrist device is not a blood test and it is not an organ scan. Nothing you wear estimates the proteins in your plasma, and nothing you wear predicts how long you'll live — anything that says otherwise is selling something.

What continuous measurement gives you is narrower and more practical: your own patterns, over months, in your own life rather than in a clinic. Whether a habit is holding. Whether the direction is right. Which of several things is moving.

That's the honest offer, and it's a good one. The research says different systems move at different speeds. Watching your own is how you find out which of yours is moving.

Not a medical device · does not diagnose or treat