“18” Is from their album “FOUR” and is the 4th track on the album. The song was announced & released on November 17th, 2014. Written by Ed Sheeran and Oliver Frank. Ed Sheeran wrote the song in the members perspective. According to an interview with Yahoo.

“Speaking to Yahoo, Sheeran confirmed that he wrote the tune with the thought of One Direction members' experiences in his head. He said: "I kinda wrote it from their perspective.”