Answer
Maria Sharapova was the last teenager to win a major singles title before Bianca Andreescu. In 2004, at the age of 17, Sharapova won Wimbledon, becoming the third-youngest woman to win a Grand Slam singles title in the Open Era. Andreescu, then 19, won the US Open in 2019, becoming the first Canadian to win a Grand Slam singles title and the first teenager to win a major since Sharapova.