A nine-year-old boy from the United States has become the youngest person to reach the summit of Aconcagua, the highest mountain in the Americas. Tyler Armstrong climbed the mountain, in the Argentine Andes, with his father and Tibetan sherpa Lhawang Dhondup. Last year, he climbed Mount Kilimanjaro, Africa's highest peak. More than 100 people have died attempting to conquer Aconcagua, which rises 22,841 feet above sea level.