While current research on the topic is not sound yet it is possible that some characateristics of ADHD do not develop until later on in life. There is also the matter of some characteristics just not being an issue until later on so the person may say nothing of them until then.
There are three main subtypes in those suffering from ADHD. The first subtype is Attention-Deficit/Hyperactivity disorder, combined type. In this type at least six or more of the symptoms that fall under the inaction category must have persisted for at least 6 months along with at least six hyperactivity-impulsivity symptoms. For children this is the most common type(American Psychiatric Association., & American Psychiatric Association, 2000). Combined ADHD type is stll makes up the largest subcategory in adults though it decreaes while the predimantly inactive type increases. The second subtype is Attention-Deficit/Hyperactivity disorder, Predominantly Inactive type. Like the combined subtype at least six inaction symptoms must be present for 6 months. It is not required that that at least six hyperactive symptoms be present. A person classified as inactive subtype does not have to be calm may appear just as high strung as any of the other individuals (American Psychiatric Association., & American Psychiatric Association, 2000). Due to the nature of this subtype it is typically harder to spot individuals who may fall under this category.