Though ADHD begins in childhood, it can change in symptom presentation as one ages, or can be undiagnosed until later on. If diagnosed later in life, they did not "just develop" ADHD. Rather, they would have had it since childhood, and it simply went undiagnosed. If diagnosed as an adult, only 5 symptoms of inattention and hyperactivity/impulsivity each need to be applicable, as found in the DSM-5. Like in children, environmental factors or other mental disorders can cause similar symptoms to ADHD, so it is important to rule these possibilities out. A thorough evaluation by a physician is recommended over self-evaluation.

Some of the more prominent symptoms of ADHD in adulthood include anxiety, disorganization, poor time management, and poor memory. Multi-tasking may be attempted (unsuccessfully).