While we normally think of depression as an adult illness, new research indicates that children as young as 3 years old can suffer from major depressive disorder, according to a study published in the Journal of Affective Disorders.

Major depressive disorder is characterized by prolonged feelings of sadness, guilt and worthlessness, trouble concentrating, sleep and appetite problems and lack of interest in formerly pleasurable things.

For the study, scientists used magnetic resonance imaging to scan the brains of 11 depressed children, whose average age was 4.5, while showing them pictures of facial expressions that represented various emotions.

Researchers found that the more severe the symptoms of depression children showed, the more activity appeared in their right amygdala-a brain structure connected to people’s remembrance of their emotional reactions to unpleasant situations. What particularly interested scientists is that the same pattern of activity also appeared in the brains of adults with depression.

In addition, research also indicated that children with depression are more likely to be depressed as adults, said Joan Luby, director of the early emotional development program at Washington University in St. Louis, the study’s author.

But treatment for depressed kids is problematic. There are concerns about how drug treatment may affect children’s developing bodies and minds. Fortunately, researchers are testing non-drug therapies to treat childhood depression. One is called dyadic play therapy, which involves kids working with their primary caregivers (with a therapist coach) to develop and regulate their emotions through structured play interactions.

If parents are concerned about a preschool child who is persistently irritable and sad and who doesn’t respond to play or fun and excitement, Luby’s advised them to remain attentive—even if their kids weren’t being disruptive. Parents don’t tend to pay as much attention to children who aren’t loud and aggressive. But these study findings show they should also be concerned about kids who are unnaturally quiet and withdrawn.

The good news, Luby said, is that “treating [depression symptoms] early may make all the difference.”

Click here to read more about the symptoms of depression.