Yiting Zhang as discussed in the comments of the MO thread, is one of the most important contemporary examples. He worked as a temporary lecturer and being aged 58 produced one of the finest works in number theory. He solved the long-standing problem infinitely re-occuring consecutive prime numbers (a weaker formulation of twin prime conjecture) [0]. This will be regarded as one of the most important breakthroughs of 21st century in mathematics. New Yorker had an in depth article about him[1]

[0] https://annals.math.princeton.edu/2014/179-3/p07

[1] https://www.newyorker.com/magazine/2015/02/02/pursuit-beauty