The diagnostic criteria of ADHD according to the international classification systems (ICD 10 or DSM IV) define the maximum age of onset of some symptoms of ADHD before the age of seven. These symptoms have to be severe and create impairments of academic or social functioning. If a child has only minor problems (compared to any other child of his age and developmental stage) the age-of-onset criterion would not be met.
ADHD is a genetically transmitted disorder. Some mothers will be able to describe symptoms of ADHD of their kids during pregnancy or in early years. But usually these symptoms cannot be distinguished from the normal development of toddlers. Usually at the time of kindergarten or the early years of school children with ADHD will present typical symptoms of ADHD because they get more problems to adapt to social rules with other children or teachers and might also have problems with their low attention span or distractibility and low impulse control.
However, children (especially girls) with the inattentive-type of ADHD might be able to compensate these problems for a longer period. So there are also some children who do not have severe impairments in school and make excellent tests. Becoming older and having higher (or different) demands of self-organisation and executive functions they might get typical problems. Some young women present typical symptoms of ADHD with hormonal changes at the age of 13 or 14. There is no general agreement among experts how to classify this type of ADHD-like symptoms if no severe ADHD-symptoms were described before the age of 7. However most doctors would recommend a trial of ADHD-treatment for these patients if they met the other diagnostic criteria of ADHD and other possible causes can be excluded.