21 Under 21 is an annual ranking by American music magazine Billboard beginning in 2010. It honours young musicians under the age of 21 in the music for impact over the previous and their potential to "rule pop culture zeitgeist over the next 12 months".
Taylor Swift topped the first ranking in 2010 at age 20. Justin Bieber (2011), Lorde (2014) and Billie Eilish (2019) are the youngest acts to be #1 (all aged 17). Willow Smith is the youngest act to appear on any ranking, (aged 9 in 2010). Smith also has the longest period of inclusion, appearing on five rankings across 11 years (2010, 2011, 2017, 2019 and 2021). Bieber; Camila Cabello; Eilish; Shawn Mendes; Lorde; Cody Simpson; Smith and Grace VanderWaal are tied for the most appearances, with five each. Bieber and Mendes are the only acts to top the ranking more than once (twice each).