Complex sentence: The youngest before Carlsen to achieve this feat was Vladimir Kramnik at the age of 25, and up until this point, only Kasparov, Topalov, Kramnik, and Anand had achieved a 2800 + rating.
Split into simpler sentences: The youngest before him was Vladimir Kramnik at the age of 25. Before Carlsen, only Kasparov, Topalov, Kramnik, and Anand had achieved a 2800 + rating.