Attention-deficit/hyperactivity disorder (ADHD) is a highly heterogeneous disorder, and the phenotypic structure comprising inattentive and hyperactive-impulsive type symptoms has been the focus of a growing body of recent research. Methodological studies are needed to better characterize phenotypes to advance research as well as clinical practice. A large U.S. population-based sample of young adults (N = 14,307, aged 17-28 years, 52.8% female) retrospectively reported their experiences of childhood ADHD symptoms. Factor analysis, latent class analysis, and factor mixture modeling of ADHD symptoms were compared to determine which underlying structure best fit the data. Fit statistics as well as substantive criteria compared models within and across model subtypes. Analyses supported a two-factor two-class structure for both male and female subjects. The two latent factors represented inattentive and hyperactive-impulsive symptom dimensions. The two latent classes divided people into a smaller affected class and a larger unaffected class. Individuals who reported having been diagnosed with ADHD were more likely to be in the affected class (OR male subjects = 4.03, 95% CI [2.65, 6.13]; OR female subjects = 5.65, 95% CI [3.15, 10.10]). This work aids in the understanding of ADHD symptomatology within the population; a majority of people experience very low symptom severity, whereas a minority of people experience high symptom severity. Within this high symptom group, however, variability in symptom experiences exists. Empirical models can be helpful in clarifying ADHD phenotypic structure that has the potential to advance research on the etiology and consequences of ADHD symptoms.