November 17, 2014
Ed Sheeran, Oliver Frank
Steve Robson, Matt Rad, Sam Miller
Lyrically, 18 describes a past relationship the narrator has not moved on from, and would like to start over again, with a more mature perspective.
On September 8, 2014, the band announced the release of their fourth album, Four, on the Today Show. On November 17, 2014, the band announced 18.
Simon Cowell is a fan of the song and talked about how he arranged for Ed Sheeran to write it. "Literally a week [after asking him for a One Direction song] Ed comes round to my house with his manager one night, turned up around 11 o’clock and said ‘Do you like this song?’ and he played me ‘18’ and it took me about a millionth of a second to say yes,”
Similar to "Strong", "Happily", "Something Great", "If I Could Fly" and "Home", fans speculate the song to possibly be about Louis Tomlinson and Harry Styles's closeted relationship. The lyric "We made a start, be it a false one I know" may be relevant to Harry and Louis moving in together in September 2011. Ed Sheeran added fuel to the fire when he remarked that he wrote the song based on One Direction themselves: "I wrote it in a hotel room in Cologne and I wrote it specifically for them. It's the first time I've done that, so it's a One Direction song rather than a 'me' song that they record. It's kind of a little bit of everything, it is ballad-y but it is up-tempo at the same time."
The lyric "We took a chance", sung by Niall Horan is pronounced as "We took a chónce" due to his accent. This became a common meme for fans online, which the band themselves caught onto during the final leg of the On The Road Again Tour, lightly teasing Niall or referencing it just before his solo. Niall was eventually asked about it in an October 2015 interview (to which he revealed he had no idea about the meme), then acknowledged it when performing the song in subsequent concerts.
The song was added to the On the Road Again Tour setlist in 2015, during the European leg.