[Poincaré's conjecture][1] has been [formulated in 1904][2], when he had just turned 50, while presenting a counter-example to another earlier conjecture of his. Probably, given the impact it has had for a whole century, the precise formulation of the conjecture can be seen as a "major discovery" by itself.


  [1]: http://en.wikipedia.org/wiki/Poincar%25C3%25A9_conjecture
  [2]: http://www.springerlink.com/content/l482x733387176u4/