Keely Age: your fitness, in years.

How it’s worked out

Across the large studies, each MET of fitness (3.5 points of VO₂ max) goes with about 14 % lower mortality, and mortality rises about 9.6 % with each year of age. Put together, a MET is worth about 1.6 years:

Keely Age = age + ln(0.86) ÷ 3.5 ÷ 0.0917 × (VO₂ max − average)
          ≈ age − 0.47 × (VO₂ max − average)

The average is the FRIEND registry’s median for your age and sex, the same tables as your fitness band, so your band and your age always agree.

For example: a man of 40 has an average of 40.1. With a VO₂ max of 52, 11.9 above it, his Keely Age is 40 − 0.47 × 11.9, about 34: 6 years younger than he is. A woman of 40 with a VO₂ max of 44 is about 33.

Why only fitness

In large studies, cardiorespiratory fitness is one of the strongest predictors of how long people live. Resting heart rate, HRV, sleep and steps matter too, but they overlap with fitness and with each other, so adding their risks together would count the same thing twice. Keely Age leaves them out of the number and shows them beside it instead, on its page, never turned into years.

How it changes

• Each Monday, from your fitness as it stood that morning. It holds through the week.
• Over six months, Keely names a change, such as “1 year younger since April”, only once your VO₂ max has moved by a full MET. Less than that is within the estimate’s noise, and it reads steady.
• What moved it is always the same one thing: your VO₂ max. The page lists the Mondays it changed, with the VO₂ max behind each.
• When it moves, Today says so once, that Monday: “Your Keely Age is 38 now”, and why.

Before it has a number

Keely Age waits for established fitness: 3 steady walks or runs that agree, and your max heart rate, seen in a hard effort or set in Settings. Until then, its card shows what’s done and what it’s waiting for, with no number and nothing guessed.

What it is, and what it isn’t

• It is a way to read your fitness against the evidence on fitness and mortality, in units you live in.
• It isn’t a forecast of how long you’ll live. It compares risks at your age; it doesn’t move a lifespan.
• It isn’t a biological age from blood or DNA, and it isn’t a diagnosis.
• It describes people like you, not a promise about you. The studies are of populations, and the link is a little weaker at older ages.

If a medicine or condition changes how your heart rate responds, Keely doesn’t estimate your fitness, so there’s no Keely Age either.

Frequently asked questions

Sources

• Fitness and mortality, per MET: Singh B et al. Comparison of objectively measured and estimated cardiorespiratory fitness to predict mortality. J Sport Health Sci 2025. doi:10.1016/j.jshs.2024.100986; and Kodama S et al. Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events. JAMA 2009. doi:10.1001/jama.2009.681
• Mortality’s rise with age: Liu Z et al. A new aging measure captures morbidity and mortality risk across diverse subpopulations from NHANES IV. PLoS Med 2018. doi:10.1371/journal.pmed.1002718
• Turning a risk into years: Brenner H et al. Rate advancement periods. Epidemiology 1993. doi:10.1097/00001648-199305000-00006; and Spiegelhalter D. How old are you, really? Communicating chronic risk through ‘effective age’ of your body and organs. BMC Med Inform Decis Mak 2016. doi:10.1186/s12911-016-0342-z
• A clinical precedent, from treadmill tests: Blaha MJ et al. Cardiorespiratory fitness and biological age. Heart 2016. doi:10.1136/heartjnl-2015-308537
• Reference averages: Kaminsky LA et al. Reference standards for cardiorespiratory fitness (FRIEND). Mayo Clin Proc 2015. doi:10.1016/j.mayocp.2015.07.026

Keely is not a medical device. Keely Age isn’t a diagnosis or a prediction for any one person.

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