Body age — also called fitness age or biological age in consumer apps — is an estimate of how old your body is behaving, based on measured signals rather than the date on your birth certificate. Healia's version combines six signals from Apple Health over a rolling seven-day window: cardio fitness (VO2 max) 30%, HRV 25%, resting heart rate 20%, sleep 10%, daily activity 10% and BMI 5%. Score better than your age and it reads younger; score worse and it reads older. It is a wellness estimate built from consumer sensor data, not a clinical measurement.
The six signals and their weights
| Signal | Weight | Why it is included |
|---|---|---|
| Cardio fitness (VO2 max) | 30% | The single strongest measured predictor of cardiovascular capacity available from a watch |
| HRV (heart-rate variability) | 25% | A window on autonomic nervous-system state and recovery |
| Resting heart rate | 20% | A lower resting rate generally reflects a heart that works more efficiently |
| Sleep | 10% | Duration and consistency underpin every other recovery signal |
| Daily activity | 10% | Habitual movement, independent of formal workouts |
| BMI | 5% | Included lightly — it is a blunt measure that says little on its own |
Each signal is scored against age-adjusted reference ranges, weighted, and combined. Healia computes this entirely on your iPhone from data already in Apple Health — no account, no server.
What moves it
- Down (younger): consistent aerobic training over months, better sleep, a lower resting heart rate, quitting smoking, moderating alcohol.
- Up (older): poor sleep, illness, heavy training without recovery, a sedentary stretch, elevated resting heart rate.
- Short-term noise: a single bad night moves it a little; it is a seven-day rolling window, so it reacts but does not lurch.
How to read it honestly
Treat body age as a motivational aggregate, not a verdict. Its real value is directional: if it trends younger over a season of training, something you are doing works. If it drifts older for weeks, that is a nudge worth acting on. It is not comparable between apps, because each uses different signals, weights and reference ranges — and it says nothing about conditions a watch cannot see, such as blood pressure, cholesterol or blood glucose.
In Healia, body age is drawn as an ink-wash tree that grows and changes with the estimate, so the number reads as something you tend rather than something you are graded on.