Complex sentence: In his youth, Thiel was a fanatical chess player; he began playing at the age of 6, and by age 12 was ranked seventh in the U.S. in the under-13 category.
Split into simpler sentences: In his youth, Thiel was a fanatical chess player. He began playing at the age of 6, and by age 12 was ranked seventh in the U.S. in the under-13 category.