Yes ADHD is considered to be a mental illness.
ADHD is a mental illness that some people have and the ADHD mental illness affects the way a person focuses on things and acts.
Someone with ADHD has a harder time focusing on things than someone who does not have ADHD.
ADHD which is a mental illness can last into adulthood although it's most commonly diagnosed in school aged children.
If a child is having a lot of trouble focusing on something such as in school studying and loses focus often then they may have ADHD.
ADHD medicine can help regain the child's focus and attention on things but should not be taken lightly.