Attention deficit hyperactivity disorder (ADHD) is a group of behavioural symptoms that include inattentiveness, hyperactivity and impulsiveness.  Common symptoms of ADHD include: 

• a short attention span or being easily distracted
• restlessness, constant fidgeting or overactivity
• being impulsive 

ADHD can occur in people of any intellectual ability, although it is more common in people with learning difficulties. People with ADHD may also have additional problems, such as sleep and anxiety disorders. 

Symptoms of ADHD tend to be first noticed at an early age, and may become more noticeable when a child’s circumstances change, such as when they start school. Cases are diagnosed in children between the ages of 6 and 12.

Source :NHS UK