"At Seventeen" is composed in the key of C major using common time and a moderate tempo of 126 beats per minute. Instrumentation is provided by a piano and a guitar. During the track, Ian's vocal range spans from the low note of G3 to the high note of A4. Some commentators connected the song to bossa nova. "Mix" magazine's Gary Eskow cited Ian's style as the opposite of Antônio Carlos Jobim's because she "explore the belly of the bossa, the flip side of Ipanema". John Lissner of "The New York Times" referred to the instrumental as having a "laid‐back bossa nova beat" and ostinato. On the other hand, AllMusics Lindsay Planer referred to "At Seventeen" as a mixture of pop rock, jazz, and blues, and music scholar James E. Perone associated it more with jazz and a "coffeehouse folksinger" approach. Perone described the song's style as more restrained compared to Ian's contemporaries. A writer for "Rolling Stone" magazine associated "At Seventeen" with "sulk-pop".