Childhood ADHD and Its Adult Mimics: When First-Time ADHD Diagnoses in Adulthood May Reflect Other Conditions—A Narrative Review

Background/Objectives: Attention-deficit/hyperactivity disorder (ADHD) begins in childhood, but its presentation changes over time. Adult ADHD is increasingly recognized; in the United States, 2023 survey data indicate that approximately 6% of adults reported a current diagnosis. This narrative review examines the developmental course of ADHD from childhood to adulthood and the controversial phenomenon of apparent late-onset presentations. Broadened diagnostic criteria, greater public awareness, and expanded access through telehealth may contribute to the rise in adult diagnoses, but these factors do not establish population-level diagnostic inflation. In some first-time adult assessments, the presenting symptoms may be better explained by an alternative or co-occurring condition; longitudinal studies also document associations between childhood ADHD and later borderline personality disorder or bipolar disorder without showing that diagnostic substitution is common. Methods: This is a narrative, non-systematic review. The primary and secondary literature were identified through targeted searches of PubMed/MEDLINE and Google Scholar (studies published through July 2026) using combinations of the terms “adult ADHD”, “diagnostic inflation”, “late-onset ADHD”, “executive function”, “borderline personality disorder”, “retrospective recall” and “stimulant diversion”, supplemented by hand-searching the reference lists of key reviews, current diagnostic manuals (DSM-5, DSM-5-TR, ICD-11) and consensus statements. All cited empirical claims and figures were verified against the primary source before submission. Results: The clinical presentation of ADHD shifts from overt hyperactive–impulsive behaviour in childhood toward executive dysfunction and subjective restlessness in adulthood. Longitudinal cohorts show both persistence and fluctuating remission, while birth-cohort evidence for apparently late-onset ADHD is substantially reduced when cases undergo repeated, multi-informant reassessment. Evidence for the recent rise in adult diagnoses is most direct for criterial broadening and expanded access, is qualitatively strong but difficult to quantify for correction of historical underrecognition and remains contested for genuinely adult-onset ADHD. Mood, anxiety, substance-use, sleep, stress-related, and personality conditions can overlap with or mimic ADHD-like symptoms. Prospective studies also report associations between childhood ADHD and later borderline personality disorder or bipolar disorder, but do not establish how often adult ADHD is diagnosed instead of either condition. Screening instruments and retrospective childhood recall therefore require interpretation within a full developmental and differential assessment. Conclusions: Adult ADHD is a genuine, historically underrecognized, and treatable condition. Its increased visibility warrants careful assessment, because a later diagnosis based on weakly corroborated retrospective evidence may be overconfident even when it is ultimately correct. Maintaining diagnostic integrity across the lifespan requires developmentally informed assessment, systematic differential diagnosis, cautious use of screening instruments, and explicit separation of clinical conclusions from administrative or medico-legal determinations.