I read this several times as C-4 not C-14 and was very curious how one would date the age of living cells with plastic explosives. I think I need more coffee.

Sorry, say what? C-14 comes from exposure to radiation also from the sun. Dating by it would make no sense in recent history because it would be too hard. Am I missing something?

Answering my own question, after rereading TFA. It's more because the C-14 was doubled and not because C-14 did not exist before.

This graph [1] available on Wikipedia answers this question. The level of carbon 14 in the atmosphere (in the southern hemisphere) roughly doubled between 1955 and 1963 ish. This coincides with the era of above ground nuclear testing. Since then it has been decaying back to the baseline.

[1] https://en.wikipedia.org/wiki/Carbon-14#/media/File%3ARadioc...

Indeed, I may have misread the GP comment, understanding that it stated that C-14 appeared because of the nuclear tests. They may have meant the addition. I did try to correct my mistake by answering my own question...