The retirement number we don't have

I’m 47, and everyone I know has a number.

Everyone has thought about the number they need to retire.

In my college WhatsApp groups, many folks are working out theirs. The ones who aren’t vocal about it have been doing it quietly. One million. Two million. Five. Ten. There are calculators for it, threads about it, and a lot of confident disagreement. People trade the figures like scores.

All of that treats money as the whole and often only game. Hit the number and you’re safe. It’s half the story at best, and it happens to be the half that’s easy to measure.

In the last year or so, as I hit my mid-40s, I started thinking more about the number I actually want.

There’s a number I care about more, and nobody in my groups is arguing about it. Not how long can I live. Not longevity. But how long can I move, think, and look after myself without quietly outsourcing my life to somebody else?

I wanted that as an age. One number. Independent until 85. At least as long as my Dad managed on his own. I spent a while trying to build it.

You can’t have it.

I liked thinking about the financial number for half of my life because that is accumulation. You play the game and you save the earnings. Independence is not something we gain. We lose it with age. That’s the difference, and that’s what makes this number a hard one to measure.

And when I tried anyway, the answer would not sit still. Run aerobic capacity forward to the age where it crosses the level below which people stop managing alone, and it swings about eight years on how you measured it, seventeen if the measurement is an estimate, and more than twenty on the decline assumption alone. That isn’t a finding. That’s a guess with a decimal point on it.

My own model, checked against an independent implementation. The arithmetic is mine; the reference data underneath it is other people's.

What you can have instead

Turn it around. Don’t ask what age you lose your independence. Ask what you need today to be on track for one.

Pick 85. Find the level of some capacity below which people stop managing on their own. Work back through the rate it declines. What comes out is a number you can check this afternoon.

The tests are ordinary. Stand on one leg with your eyes closed. Get down to the floor and back up without putting a hand down. Walk a measured stretch at your normal pace. Stand out of a chair five times. Count your push-ups. Take your pulse, measure your waist. Two more outdoors if you own a watch that reads heart rate.

Nothing to buy and nothing to book. A chair, a tape measure and a patch of floor. Every one of them has published numbers behind it for someone your age and sex, and some of those numbers are a great deal better than others.

Walking speed is the one with the most behind it. 1.20 m/s is roughly what it takes to cross a road on the signal. To still be above that at 85, a 47-year-old man needs 1.63 m/s today. The published median at 47 is 1.43.

Gait speed: Bohannon & Williams Andrews, Physiotherapy 2011 — 51,248 people across 79 studies. Crossing threshold: Western et al, Age Ageing 2025.

So the average 47-year-old man is already behind. Not ill. Not diagnosable. Nothing a doctor would raise at a physical. Just behind. And nobody tells you, because “normal for your age” is the only feedback you ever get, and it isn’t the same claim as “on track”.

Two numbers

I’ve had a retirement number for years. I know where I stand against it.

Now I’ve got the other one, and it’s the one that decides whether the first was worth the trouble.

Fifteen minutes, a chair and a tape measure.