Complex sentence: At the age of 20, he was hired by Captain Meriwether Lewis for the United States' official expedition into the newly acquired Louisiana Territory, which departed St. Louis, Missouri in early 1804.
Split into simpler sentences: At the age of 20, he was hired by Captain Meriwether Lewis for the United States' official expedition into the newly acquired Louisiana Territory. The expedition departed St. Louis, Missouri in early 1804.