ABSTRACT Research has sought to understand insomnia through identification of subtypes, yet age of onset has received limited focused empirical attention. This investigation aimed to detect clinically distinct age of insomnia onset subgroups utilising latent profile analysis (LPA). Participants were 618 adults, aged 18–71 years ( M = 28.94, SD = 11.06), with insomnia. Participants completed a survey assessing insomnia natural history and causal attributions; sleep disturbance and impairment; pre‐sleep arousal and threat monitoring; stress, mental and physical health; and social functioning. LPA was performed on age of insomnia onset. Binary logistic regression analyses were performed to evaluate associations between clinical measures and early versus late onset insomnia with statistical adjustment for chronological age and sex. Results showed a two‐class model (Class 1: n = 547, 88.5%, M onset = 19.21 years, range = 0–34 years; Class 2: n = 71, 11.5%, M onset = 43.49 years, range = 35–68 years) was optimal for forming insomnia age of onset subgroups. Bodily arousal and developmental (e.g., childhood experiences, traumatic events) contributors to insomnia onset, greater overall and cognitive pre‐sleep arousal, later bedtime and rise time, greater depressive symptoms, and endorsement of lifetime major depressive disorder, migraine, and arthritis were significant indicators of early onset insomnia subgroup membership. Hormonal contributors (e.g., ageing, menopause) to insomnia onset and maintenance, and more positive global mental health were significant indicators of late onset insomnia subgroup membership. Findings suggest the relevance of mindfulness‐based, acceptance‐based, and trauma‐focused adaptations of cognitive‐behavioural therapy for early onset insomnia, and management of ageing‐related hormonal changes for late onset insomnia.