# Half the Half Centurians

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If I make a statement "half the humans that lived to be 50 years old are alive today, and the other half are deceased" I am most likely off one way or another.

1. On which side am I off?

2. What age should I replace "50 years old" with, to make the statement correct?

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The median life expectancy is what I'd look up.

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Very good questions - there is an episode of the BBC radio programme More or Less that settles the old chestnut that more people are alive at moment than have already lived and died (out by at least an order of magnitude - perhaps dead outnumber living by x20 or so). I will dig it out - but IIRC the estimations and very broad assumptions even in that simpler calculation were pretty heavy-duty.

I would not be at all surprised that if you were to be systematic and account for all your errors and guesses your answer would be so vague as to be meaningless unfortunately.

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I'd expect it to be higher, because it's heavily weighted by child mortality. Median life expectancy may have been 40-50 years not long ago, but life expectancy if you made it to 1 year was significantly higher, which means plenty of people made it past 50.

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• 5 months later...

This site says 20% of all humans who ever lived past age 65 are alive today:

http://www.trinity.edu/mkearl/ger-biol.html

So I suspect you have to go a lot higher than age 50, perhaps age 90 or 100.

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122.

There's a man that Bolivian records claim is 123 (if you trust Bolivian records) and the oldest person with confirmed documentation is a now-deceased 122 year old.

Assuming no one else has lived to 122 (and I think that is probable) and the Bolivian man is actually 123 (somewhat less probable, but still possible) then the answer is 122.

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