Body age

1

What does Body age show?

The gap, in years, between your calendar age and the age your recent days resemble. It is an estimate.

Body age opens with one gap in years: younger or older than your age by the calendar. If the gap rounds to nothing, it says Right at your age. Around it, a flower has one petal for each part of your day.

The sheet under the flower has four sections. Over the months draws your age week by week, from your second Monday. Your next step names the one thing worth doing now. Your parts lists the six parts. Confidence says how sure Aera is.

2

What goes into it?

Signals your watch and phone record, grouped into parts of your day. A cuff and a tape can be added.

Seven signals count. Fitness is your Cardio fitness, which is VO2 max. Movement is Active minutes and Daily steps. Heart is Resting heart rate and HRV. Sleep is Time asleep and Sleep regularity. Strength and Walking pace show as petals too, but they stay out of the age.

Most signals are the middle value over a recent window of days: 28 days for Cardio fitness, Resting heart rate and Time asleep, and 14 for HRV and Daily steps. Active minutes are an average week over 28 days. Sleep regularity comes from its own score.

A signal with few days behind it, or an old newest day, counts for less.

Active minutes come from your Apple Watch's exercise time on the days it measured it. On other days Aera uses the workouts you logged, from any source, which miss brisk walking and everyday effort, so Aera trusts them less, down to half as much when no day came from the watch. Half is Aera's judgement. Rest days count as zero.

3

How do the signals become one age?

Each signal becomes an age, the ages are blended by weight with your real age as an anchor, and the result is capped.

Each signal is turned into an equivalent age: the age at which people typically show your level. Cardio fitness, HRV and Daily steps are read against typical levels for your age and sex. Resting heart rate is read against a fixed young-adult level. The rest use lines that are the same for everyone. Why each matters is told in VO2 max, Resting heart rate, HRV, Steps, How much sleep you need and Bedtime, regularity and daylight.

Where a study says how much risk changes, Aera turns that into years with its own rule: risk that doubles counts as 8 years older. That rule is Aera's choice, not taken from a study.

The signals are blended by weight. Cardio fitness weighs most, then Active minutes, Resting heart rate, Time asleep, Sleep regularity and Daily steps, with HRV last because it is noisy. Aera set these weights itself, not taken from a study.

For Active minutes, a pooled analysis of cohort studies, with over 30 million people, linked more activity with lower risk of death and disease. The difference was biggest between no activity and the recommended 150 minutes a week.¹

Aera takes 150 minutes a week as neutral. Each 100 minutes more counts 2.5 years younger, each 100 fewer 2.5 years older, up to 3.5 either way. Those figures are Aera's choice.

Your real age joins the blend as an extra voice, so a read built on little data stays near your age. The blend is eased toward a limit of 9 years either way, and never shows under 18. Both limits are Aera's choice.

The watch's VO2 max is multiplied by 1.10 before it meets treadmill norms, as told in Fitness trends.

4

How does the age settle week by week?

Each Monday Aera reads your recent days again and blends the last weeks, so one odd week barely moves it.

Each Monday at nine in the morning, Aera reads your recent days again. Until then you see last week's age.

The age you see averages the last 6 weekly reads. The newest week counts most, and each older week about two thirds as much as the next newer week. The six weeks and the weights are Aera's choice, not taken from a study.

5

What do a cuff and a tape add?

Home blood pressure and waist to height can be added. They count on top of the watch signals and never raise confidence.

You can add a cuff and a tape. The cuff is your newest week of readings, picked as on the Blood pressure average page but from the last 30 days, not 14. It counts once those days hold readings on 3 days. The tape is your waist at your belly button, set against your height from Apple Health, or one you type in the setup.

Both count on top. Each adds its years at its own weight but takes no seat in the blend, so a reading at the ideal moves nothing, and neither changes how sure Aera is. A cuff can take at most a year off, and only under 70. A tape can only add years. Both weights are Aera's choice.

Why a cuff? A pooled study of 17,921 people in 17 cohorts measured blood pressure at home or with a daytime monitor. Risk rose steadily from levels as low as 110/65. For each 20 points on the top number, risk of dying was 1.42 times higher in people aged 60 or younger, but 1.09 times higher in people over 80.²

So the relative risk falls with age, while the starting risk climbs. Aera follows the fall: per 10 points on the top number, its cuff counts about 2 years at age 60 and about half a year at 80, with a straight line between, which is Aera's choice.²

Low pressure in old age is different. Among 415,980 people over 75, pressure under 130/80 went with more deaths, and high pressure did not predict deaths over 85. Aera sets its own lines: no credit for low pressure from age 70, and blood pressure stops counting at 80.³

Why a tape? A review of 72 cohort studies, with about 2.5 million people, found each 0.1 higher waist-to-height ratio went with 1.24 times the risk of dying from any cause, even after accounting for body mass index. The curve was nearly J-shaped.⁴

Aera counts about 2.5 years for each 0.1 above a ratio of 0.50, and gives no credit below it. The 0.50 line is Aera's choice.

6

How sure is the age, and when does it wait?

Aera shows how sure it is as a percent, marks and a give-or-take band. Without a birthday, sex or enough wear, it waits.

Aera needs your birthday and sex before it can compare you with anyone. Until you add them, the page says Your body age starts with you. With both but too few days of wear, it says Still learning you. A cuff or a tape alone never makes an age.

The next-step card says Wear your watch for three weeks while fewer than three weekly ages exist, or while confidence is under 45 percent.

Confidence is a percent with up to four marks beside it: four from 80 percent, one below 45. The percent weighs how many of your watch signals have data and how well recent days cover them. It also weighs how many days you have worn the watch, counting up to 90, and how many of the last 7 had data. The mix is Aera's own, not taken from a study.

Give or take is the band either side of your age. It is never under 1 year, and it narrows as your days of wear and your confidence rise.

7

What can't it tell you?

How long you will live, or how old your cells are. It compares your recent habits and vitals with norms.

Body age is not one of the clocks scientists build from blood or DNA. It reads your watch, and a cuff and a tape if you add them, against norms and risk curves from studies of groups. Those clocks are told in Biological age.

Your next step is a what-if. Aera runs the same sums with one signal moved, such as 90 more active minutes a week, and shows what that would have done to this week's age. It is not a promise.

Some things a watch cannot see. It does not know how hard you lifted, so Strength stays out of the number. Walking pace is read against your own usual, not against people your age, so it stays out too. A part with no reading never counts against you.

Body age is an estimate. If you have symptoms, or a number worries you, talk to a doctor.