One in five American young adults under the age of 25 lives with a mental illness or behavioral disorder, NBC reports, citing the Centers for Disease Control and Prevention.

The big picture: Per the CDC's research, the state of young adult mental health in the U.S. is only worsening, with the suicide rate among teenage girls reaching a 40-year high in 2015. Here's a look at the numbers that tell the story of this crisis.

The numbers:

• 15 million, or 1 in 5, American children and young adults battle mental illnesses or learning disorders.
• 10 million, or two-thirds of them, are undiagnosed or aren't receiving treatment.
• Among children ages 3 to 17: 6.8% are diagnosed with ADHD, 3.0% suffer from anxiety, 2.1% suffer from depression