What is the age of youngest pop star?

As of my last update in September 2021, the youngest pop star to achieve mainstream success and widespread recognition is Billie Eilish Pirate Baird O'Connell, known simply as Billie Eilish. She was born on December 18, 2001, making her about 20 years old.

Billie Eilish gained significant attention at a young age with her debut single "Ocean Eyes," which she released in 2015 when she was only 13 years old. Since then, she has risen to become one of the most influential and successful pop artists of her generation. She has won numerous awards, including multiple Grammy Awards, Brit Awards, American Music Awards, and Billboard Music Awards.

Please note that there may have been younger pop stars that emerged after September 2021. To stay up-to-date, I recommend checking reputable entertainment news sources for the latest information about young pop stars.