Katie Ledecky

From the minute Katie Ledecky took the Olympic stage at the age of 15 at the 2012 games in London, she took over the sport of swimming. It shouldn't have been much of a surprise, as Ledecky was a virtuoso in the water at a young age, but the expectation was that she would compete with the world's best, not dominate with the second-fastest time ever in the 800-meter freestyle and a large four-second lead over second place. After her success, she went back to high school, where she set national records for the 500-yard freestyle and 200-yard freestyle. In 2013, she competed in the World Championships in Barcelona, earning four gold medals and setting the world record for the 1500-meter freestyle and 800-meter freestyle. By the end of the year, Ledecky was crowned the World Swimmer of the Year at the age of 16.

She didn't slow down, though, and promptly beat the world records she set in the 1500-meter and 800-meter freestyle in 2014, as well as notching a third world record in the 400-meter freestyle. She also earned five more gold medals at the Pan Pacific Swimming Championships. When 2015 rolled around, Ledecky once again reset more of her own world records and notched another five gold medals. Her total dominance of the sport of swimming led her to being named to the Time 100 list in 2016, becoming the youngest person on the list.