When children or teenagers act out, are irritable or isolate themselves, experts say parents should not always assume it’s just a phase. Mental, behavioral and developmental disorders begin in early childhood, and one in six children ages 2 to 8 years old in the United States has been diagnosed with one of those disorders, the Centers for Disease Control and Prevention reports. On top of that, 50 percent of all lifetime mental illness begins by the age of 14, the National Alliance on Mental Illness says.