What are ADHD symptoms and signs?

The diagnostic criteria for ADHD are outlined in the Diagnostic and Statistical Manual of Mental Health, 4th ed. (DSM-IV). All of the symptoms of inattention, hyperactivity, and impulsivity must have persisted for at least six months to a degree that is maladaptive and inconsistent with the developmental level of the child.
Inattention:

• The child often fails to give close attention to details or makes careless mistakes in schoolwork, work, or other activities.
• The child often has difficulty sustaining attention in tasks or play activities.
• The child often does not seem to listen when spoken to directly.
• The child often does not follow through on instructions and fails to finish schoolwork, chores, or duties in the workplace (not due to oppositional behavior or failure to understand instructions).
• The child often has difficulty organizing tasks and activities.
• The child often avoids, dislikes, or is reluctant to engage in tasks that require sustained mental effort (such as schoolwork or homework).
• The child often loses things necessary for tasks or activities (toys, school assignments, pencils, books, or tools).
• The child is often easily distracted by extraneous stimuli.
• The child is often forgetful in daily activities.

Hyperactivity:

• The child often fidgets with his/her hands or feet or squirms in his/her seat.
• The child often leaves his/her seat in the classroom or in other situations in which remaining seated is expected.
• The child often runs about or climbs excessively in situations in which it is inappropriate.
• The child often has difficulty playing or engaging in leisure activities quietly.
• The child often talks excessively.

Impulsivity:

• The child often blurts out answers before questions have been completed.
• The child often has difficulty awaiting his/her turn.
• The child often interrupts or intrudes on others (for example, butts into conversations or games).

DSM-IV criteria for diagnosis of ADHD requires that some hyperactive, impulsive, or inattention symptoms that cause present difficulties were present before 7 years of age and are present in two or more settings (at school [or work] or at home). Similarly, there must be clear evidence of significant impairment in social, academic, or occupational functioning. In addition, symptoms may not entirely be caused by another severe physical disorder (for example, severe illness associated with chronic pain) or mental disorder (for example, schizophrenia, other psychotic disorders, severe disabling mood disorders, etc.).
Inattention symptoms are most likely to manifest about at 8 to 9 years of age and commonly are lifelong. The "delay" in onset of inattentive symptoms may reflect its more subtle nature (vs. hyperactivity) and/or variability in the maturation of cognitive development. Hyperactivity symptoms are usually obvious by 5 years of age and peak in severity between 7 to 8 years of age. With maturation, these behaviors progressively decline and often have been "outgrown" by adolescence. Impulsive behaviors are commonly linked to hyperactivity and also peak about 7 to 8 years of age; however, unlike their hyperactive counterpart, impulsivity issues remain well into adulthood. Impulsive adolescents are more likely to experiment with high-risk behaviors (drugs, sexual activity, driving, etc.). Impulsive adults have a higher rate of financial mismanagement (impulse buying, gambling, etc.).