Songfacts®:

• John and TJ Osborne wrote this lament to lost love with Craig Wiseman (Blake Shelton's "Hillbilly Bone," Tim McGraw's "Live Like You Were Dying"). We hear the brothers lament the love lost.
  
  How yah been I'd love to hear bout how you been
  You're the only broken heart I've ever had and love to have again
  
  "It's that eerie nostalgia that you can't quite put your finger on," guitarist John Osborne told Billboard magazine. "It's about looking back on a time in your life, regardless of how a previous relationship went. Whatever went wrong seemed to drift away and you only remember the glow of it, and the beauty of it and the importance of it."
• The Jay Joyce-produced track was the third single to be dropped from Pawn Shop. The release has a slightly different feel to the version that the pair first recorded. "Jay had the idea for the song, and it was kind of weird because people loved the original version we had, and we knew people were probably not gonna like the fact that it was changed," said TJ Osborne. "He's like, 'I think it would be really cool to have a song that was a little more, felt like you could put on and it'd just be chill.'"
  
  "A lot of country records don't really have that; they're a little bit more kind of attention grabbing," he added. "And so, we loved that idea. We have those songs that are kind of a little bit more in your face, but it'd be cool to have a song that was just very relaxed and kind of almost have that kind of California country thing to it."
• The Osbornes added their voices to the backing vocal group which also includes their sister, Natalie Osborne; bassist Pete Sternberg; and John's wife, English singer-songwriter Lucie Silvas. The latter clocked up a couple of Top 10 hits of her own in her native country in 2004.
• Osborne John and TJ Osborne partnered with up-and-coming director Justin Clough for the video, which documents a young boy who falls in love with a girl who has a dark past.
  
  "We wanted to take a very particular story that's quite left of center and apply it to the lyrics of '21 Summer,'" said John Osbourne. "And the fact that a young boy ends up with a girl that's far more experienced than him and leads a darker life than him and they have something that actually shows him how to grow up and be a man, you know, and what it's like to deal with heartache and deal with frustration and something seemingly catastrophic then turns into a very important part of your life."
  
  TJ Osbourne added: "We wanted a storyline that was equally, when you saw it, I think you can have your own relatable experience that would I think probably initially, but then it really was thinking of how do we make a storyline that when you watch it, it really does make you really feel that heartbreak, not only for you, but for that person in the video experiencing that same type of thing. So, [we're] just really trying to evoke that emotion and get it out and make it feel real and honest."
• Fans assumed for many years that "21 Summer" is about a girl who got away, but when TJ Osborne came out as gay in February 2021, it threw a different light on the lyrics.
  
  It turns out TJ is singing of splitting up with a guy. He explained to Time it was his first heartbreak in his early 20s and what made it worse was he felt like he couldn't tell anyone. "I was mad that no one knew why I was hurting," he said.
  
  The loneliness TJ felt as he went through the breakup on his own has remained with him. He recounted that it's still tender for him years later: "There are so many times I've sung that song and wanted to cry. People love that song, but the emotion of it is deeper than they even realize."