Simplify the following text for a Grade 8 native English-speaking person.
Original: GÃ del is best known for his two incompleteness theorems , published in 1931 when he was 25 years of age , one year after finishing his doctorate at the University of Vienna .
Simplified: The theorems were published in 1931 . He was 25 years of age , and had just finished his doctorate at the University of Vienna one year earlier .