Is life really halfway over by 23?

I remember seeing this tweet from Sam Altman (OpenAI founder) a while ago. Stopped me in my tracks:

Essentially, time feels like it moves faster as you get older. Because when you’re a kid everything is new; on your 2nd birthday a year is 50% of your life, but at 92 a year is only 1.1% of your life (here’s a really cool website to visualize this).

So I think Sam’s assertion is valid, by 23 you’ve more or less perceived half of the things you’ll ever perceive. It’s a haunting realization. His takeaway is also beautifully correct: “Don’t waste time”

Recently I came across the tweet again and it got me thinking — this ‘halfway by 23 thing’ kind of assumes a modern life expectancy of ~80 years, right?

But what about back in time when life expectancy was much lower? and when the speed of societal / technological change was much slower? It sent me down quite the rabbit hole, and the results surprised me. Allow me to indulge for a moment:

Life Expectancy vs Subjective Halfway Point Over Time

I went back and had Claude research this question for me:

First, a clarification: historical life expectancy stats are apparently wildly misunderstood. People didn't typically die at 35 in Medieval times (which I thought they did); that's just an average dragged down by catastrophic infant mortality until recent history. If you survived childhood, you could easily reach your 60s or 70s.

I had Claude research life expectancies across different eras and calculate the subjective halfway points using a mathematical model where time perception is inversely proportional to the square root of age. I tried to have it also account for the accelerating nature of society/technology which in theory would make time feel like it’s moving even faster. Here’s the results (I had Claude plot them for me):

So cool. You can see some interesting things here:

1. Life expectancy has nearly tripled from Ancient times (30yr) to present (83yrs) which is almost incomprehensible

2. When you adjust for infant mortality and take expectancy from age 25, the increase is more gradual (+60% from 53 → 83), though still significant

3. Despite this huge increase in expectancy, the subjective “halfway point” of life has remained astonishingly stable (around 20-25yo) throughout human history

Whether you lived in Ancient Rome or Renaissance Florence, by your early twenties, life subjectively felt half over - even though we live on average 60% longer. How is this possible??

This second chart explains why — like I eluded to earlier, a year at age 20 feels only about 70% as long as it did at age 10. By 40, a year subjectively shrinks to about half its childhood length. The math simply works out that regardless of whether your lifespan is 50 or 80 years, you'll hit that 50% subjective mark in your early twenties:

This tickled my brain. So I decided to take it a step further — how would this phenomena evolve into the deep future? Let’s see:

Extrapolation Out: Expectancy & Halfway in 1000 Years

This is where things get interesting. If we project these trends forward — imagining lifespans extending to 150, 250, or beyond as biotechnology advances — i think we might see a reversal of the historical pattern.

In a "post-human" era (where people live to 250 (perhaps around year 3000)), the subjective halfway point could actually decrease to age 15. With such dramatically extended lifespans, the overwhelming majority of subjective experience would be compressed into the early decades, followed by centuries of increasingly rapid time perception.

Your last 50 years would forseeably go by in the blink of an eye (like that Kenny Chesney song on steroids). Here’s my revised chart with 1,000 y eras of projected future data:

This raises fascinating questions: would our brains adapt?

Would neural enhancements allow us to experience novelty differently? Or would our 20s still feel like subjective midlife, creating a strange psychological tension where you're objectively only 8% through your life but subjectively half-finished?

Or maybe we’ll invent some Limitless pill that allows us to access “100% of of brain” and bend time to whatever intervals we prefer.

Takeaways: A Life Worth Living

After digging this 50-foot rabbit hole in my evening yesterday I wasn’t quite sure what to do with it. These finds leave me a feeling a bit fuzzy.

First, I think the “25 is halfway” constant throughout all of human history says something fundamental about our psychology. We naturally front-load a lot of our lived experience.

This might explain the quarter-life crisis many people experience in their midtwenties. Like you really are ‘halfway’ in the only way that matters to your brain. It’s more than natural, it’s mathematical baby.

Are there ways around this subjective speed limit? Absolutely - you can yearn to never stop learning, never stop experiencing. But the older you get, the harder it inherently is to turn the flywheel of novelty. (There’s a beautiful podcast about this from Radiolab, I’ve re-listened 4-5x).

Second, it makes me think that attempted at extending lifespan won't necessarily give us more subjective time. The Brian-Johsonites’ attempts at staying forever young may be futile

Unless of course we also transform how our brains process novelty and form memories. Which could be entirely possible. But until then, Sam's advice remains timeless: don't waste time—especially those precious early years that count for so much more in your internal experience.

Peace,
Ramsey