Background: Young maternal age is one of the numerous risk factors for delivery before 37 weeks of gestation, yet the mechanisms are unclear. The purpose of the current study was to investigate the association between teenagers and the risk of preterm birth (PTB) in a large and recent cohort study. Methods: We conducted a population-based retrospective cohort study using 2007–2011 California birth certificate records linked with hospital discharge indices and United States census data for nulliparous 13–20 year olds who gave birth to singletons. Maternal age was examined categorically at 1 year intervals. PTB was defined as delivery at <37 weeks of gestation with further distinction between <32 and 32–36 weeks, and between spontaneous and medically indicated deliveries. Adjusted multivariable logistic regression was used to estimate odds ratios (OR) for PTB. Results: The prevalence of PTB was highest among the youngest (13 year olds, 14.5%) and lowest among the oldest (20 year olds, 6.7%). After adjusting for maternal and paternal race/ethnicity, paternal age, initiation of prenatal care, source of payment, pre-pregnancy body-mass-index (BMI), height, smoking, and poverty; young mothers of ages 13, 14, 15, and 16 years had increased odds for spontaneous PTB at <32 weeks [OR (CI): 3.76 (1.83–7.75), 1.65 (1.10–2.48), 1.55 (1.24–1.93), 1.19 (1.00–1.42), respectively] compared to 20 year olds. All teenagers, excluding 19 year olds, had elevated odds of spontaneous PTB at 32–36 weeks. Conclusions: Nulliparous teenagers were at increased risk for spontaneous PTB, especially those 16 years or younger. Medically indicated PTB was not associated with young age.