It is important to note that the inattentive symptoms are more consistent across the lifespan, whereas, impulsivity and hyperactivity may become internalised or manifest differently in older adolescents and adults.
ADHD was once considered to be a disorder of childhood, where presumably the individual’s problems dissipated upon the advent of their 18th birthday. However, impairments findings suggest that while symptoms of inattention persist with age, those associated with hyperactivity and impulsivity are internalized and/or manifest differently. The process by which, and reason why, this occurs remains a mystery. It could be attribute it to maturation of neuronal mechanisms, or the result of changes in environmental demands are to blame. However, what is known is that the typical symptoms of hyperactivity are of little to no use for diagnosis in adults (Barkley, 2006).
It is not that they disappear per say, rather they manifest differently.
For example, the hyperactive child who is constantly in motion, may become the chronically restless adult, who cannot wait for others, speeds while driving, and is uncomfortably doing anything slowly and systematically. Overt motor activity may become verbal hyperactivity; they may often blurt things out and make inappropriate comments. Another issue is the child who impulsively leaves their seat during class, may become the reckless adult, who quits their job on a whim, spends money without thinking, is unprepared for meetings, or has difficulty resisting temptation.
Authors of the article: Kate Carr-Fanning and Conor Mc Guckin, the School of Education, Trinity College Dublin, 2012