Bernhard Riemann
There is a well-known story from Riemann’s school days.  The school director excused him from maths lessons because the lessons were much too easy for him.  Bernhard asked the director whether he could borrow a difficult maths book to read, so the director lent him Théorie des Nombres (Theory of Numbers) by Legendre.  This was a huge book with maths that was so difficult that only a few people in the world would have understood it all.  The director was disappointed when the boy brought the book back after only six days.  He asked him how far he had got.  The boy said he had read the whole book.  This was true, and he had understood and remembered it all.  Later in life, when Riemann was 33, he developed the famous Riemann hypothesis.  This was an article that was only 8 pages long, but he developed his idea from what Legendre had written.  Mathematicians ever since have struggled to try to prove what Riemann wrote.