Mary Rose O’Leary of Eagle Rock, Calif., has shepherded three children into adulthood, and teaches art and music to middle-school students.

Despite her extensive personal and professional experience with teens, she admits she’s often perplexed by their behavior.

Teenagers can be volatile and moody. They can test your patience, push your buttons and leave you questioning your sanity — and theirs.

Mental health challenges are a serious — and growing — problem for teenagers: Teen and young-adult suicide has nearly tripled since the 1940s. The rate of 12- to 17-year-olds who struggled with clinical depression rose by 37 percent in a decade, according to a recent study.

And schizophrenia and other psychotic disorders often manifest themselves in adolescence.

In fact, half of all mental health conditions emerge by age 14, and three-quarters by 24, said Dr. Steven Adelsheim, director of the Stanford Center for Youth Mental Health and Wellbeing, part of the university’s psychiatry department.

For parents, it’s often hard to separate the warning signs of mental illness from typically erratic teenage behavior.

When O’Leary’s son, Isaac, now 23, was a teen, he had two run-ins with police — once for hosting a wild party while his mom was away, and again when he and a friend climbed up on the roof and challenged each other to shoot BB guns.

O’Leary dismissed those incidents as teenage pranks. But she did start to worry when she was in the midst of divorce proceedings and noticed that Isaac started exhibiting some unusual behavior. He complained of stomachaches and racked up absences from school.

That’s when she decided that it was time for the family to see a therapist. “It’s a question of what’s normal for my kids,” she said.

Know the baseline

Mental health experts say the first step in recognizing possible mental illness in your children is to know their habits and patterns — to spot when they deviate from them — and to create an environment in which they feel comfortable talking with you.

Instead of asking your teen to talk, share an activity that will give your child the chance to open up: Cook dinner together, walk the dog, take a drive, said Tara Niendam, an associate professor in psychiatry at the University of California-Davis.

“You just want to know how they’re doing as a person. How are things going at school? How are their friends? How are they sleeping?”

As part of getting to know your teen, monitor and limit your child’s social media activity, said Dr. Amy Barnhorst, vice chairwoman for community mental health in the UC-Davis psychiatry department.

“Social media gives us this important window into what’s going on in teenagers’ lives,” she said.

Once you know your child’s baseline, you’ll be more attuned to signs of mental illness: persistent changes in your child’s everyday life that last more than a week or two.

Be aware of disruptions in sleep, appetite, grades, weight, friendships — even hygiene.

Maybe your son is spending even more time alone in his room. Perhaps your daughter, who is particular about her appearance, stops wearing makeup and isn’t showering.

“It’s really when you see kids falling off the curve in every sphere of their lives,” Barnhorst said. “They’re having problems with their academics, problems with their family, problems with their friends and problems with their activities.”

Essentially, take note when “there’s a lot of shifting and chaos” in their lives, she said.

Remember, you’re looking for changes in many aspects of your child’s life that last for a few weeks, not the typical — but temporary — sadness that comes with a breakup or the unfortunate mouthing off you get when you ask your kid to clean his or her room.

Seeking help

If your child still has the same friends and is participating in the same activities, unpleasant behavior “is not necessarily something to worry about,” Barnhorst said. “That could just be teenagers going through growing pains.”

But some behavioral changes could indicate a deeper problem. For instance, teenagers with depression may be more irritable than usual, Adelsheim said. They might snap at friends or even the family dog.

When you become worried that your child’s behavior may indicate something more serious, offer your child love and support — and seek help, experts say.

(And avoid phrases like “What’s wrong with you?” and “Snap out of it” when talking with your kids, Niendam said.)

If your child threatens suicide, or you think he’s in imminent danger, take him to the emergency room.

If there’s no immediate danger, start with your child’s pediatrician or primary care physician. In some cases, the pediatrician will be able to address the problem directly — or may refer you to a mental health specialist.

This is where it could get tricky. You may face a long wait for a specialist — especially if you live in a rural area — and may find that many aren’t accepting new patients. Barnhorst suggests calling your health insurance plan and asking for a list of in-network therapists, psychologists and psychiatrists. Then hit the phone and hope for the best.

“One of the most serious problems we have in this country on the mental health front is the lack of access to care,” said Dr. Victor Schwartz, chief medical officer of the Jed Foundation, a New York-based organization that works to prevent suicides in teens and young adults.

While you’re seeking medical help, don’t forget to contact your child’s school, which may be able to make accommodations such as offering your child extra time for testing, Niendam said.

She also suggests connecting with your local chapter of the National Alliance on Mental Illness (nami.org).