"21" is a song recorded by American singer Hunter Hayes as the lead single for his third studio album, The 21 Project (2015). Written by Dallas Davidson, Kelley Lovelace, and Ashley Gorley, the song is Hayes' first to not feature an official writing credit. It was released in the UK on May 21, 2015 before being released to digital retailers in North America on May 26, 2015. It is also included on his second extended play, 21 (2015). "21" impacted country radio on June 8, 2015 and entered the Billboard Country Airplay chart at number 32.

"21" is a mid-tempo country pop and pop rock song about the "carefree" days of being a young adult. The staff at country music blog Taste of Country noted the song's lyrics as being more superficial and straightforward than those on his previous album, Storyline (2014). A chant of "nah nah nah" opens the song, over a guitar riff that Hayes added as part of his effort to personalize the song, and is repeated again at the end of each chorus. It has been described as a "party song" by critics including Brian Mansfield of USA Today.

The song was written by Dallas Davidson, Kelley Lovelace, and Ashley Gorley, although Hayes made changes to the arrangement with Davidson's approval.Billboard credits him as a songwriter for his contributions. Hayes also co-produced the song with Dann Huff.

On May 11, 2015, Country Aircheck published a full-page ad online with a black-and-green calendar bearing Hayes' official logo and highlighting the date May 21, leading to speculation that his then-upcoming single would be released on that date. Hayes continued promoting the single by posting twenty-one consecutive tweets containing only solid-black images, alluding to the song's title. He confirmed the title on May 18 and 19, by posting a black square with a green 2 on the left half and a green square with a 1 on the right half, respectively, on those dates. On May 21, 2015, the song was released to streaming services and to iTunes UK, and was made available to country radio. It was released to digital retailers in Canada and the United States on May 26, 2015. The song official impacted country radio on June 8, 2015.

...
Wikipedia