In terms of chords and melody, I'll Sleep When I'm Older is more complex than the typical song, having above average scores in Chord Complexity, Melodic Complexity, Chord Progression Novelty and Chord-Bass Melody.
I'll Sleep When I'm Older is written in the key of D Major. According to the Theorytab database, it is the 2nd most popular key among Major keys and the 2nd most popular among all keys. Major keys, along with minor keys, are a common choice for popular songs. The three most important chords, built off the 1st, 4th and 5th scale degrees are all major chords (D Major, G Major, and A Major). See the D Major Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!