“21” is the lead single from Hunter Hayes’s third studio album. It was released to country radio on May 21, 2015.

According to his interview with Bobby Bones:

It’s about the spirit of a feeling, of freedom. It talks about going back to your 21st birthday.“ He adds, "It’s all about that feeling of excitement and anticipation of whatever’s next, and just happiness. It’s a celebration, it’s a celebration of life, and it’s really fun to sing live too!

Note that the song’s about him partying like he’s 21. He was 23 when he released it.

Listen to the song here.