The narrative suggests another reason for the "legend."

He wrote a manuscript spelling them out when he was 20, the night before he was killed in a duel. A Norwegian mathematician and contemporary of Galois's named Niels H. Abel died of tuberculosis at age 26 after solving a 300-year-old problem and discovering what are now known as Abelian functions.

Life expectancy has increased dramatically in the last century, so results from before that would be skewed young.

I talked with David Goodstein at Caltech once about the youth legend in physics. His basic view was that people took two data points---Newton and Einstein---and over-extrapolated. He pointed to Feynman (among others) who continued to make major contributions throughout their careers. Of course, a couple counterexamples isn't enough to disprove a trend, but it does seem like the legend is build on a rather flimsy foundation.

N.B. I found myself quite enjoying the writing in this article, and I realized that the author was a colleague of mine from college. Lila Guterman was a writer for the Harvard Science Review when I was its editor-in-chief. I particularly recall a bang-up article she wrote on naked mole rats. Glad to see she's keeping up the good work!

I have a feeling that the Field's Medal requirements help drive down the age for Mathematicians a lot more than the article gives it credit.

I wish the article proper had treated fields other than mathematics.

(Though that won't keep me from exclaiming: Woohoo! Go Geology! I still have a chance!)