Claim: Jurjen Ferdinand Koksma was 26 when he became a professor at the Vrije Universiteit Amsterdam where he became the first professor in mathematics for the university.
Evidence: Jurjen Ferdinand Koksma (21 April 1904, Schoterland – 17 December 1964, Amsterdam) was a Dutch mathematician who specialized in analytic number theory. Around the same time, aged 26, he was invited to become full professor at the Vrije Universiteit Amsterdam. He accepted and in 1930 became the first professor in mathematics at this university.