‘Prison Break’ star Wentworth Miller, 49, shares autism spectrum disorder diagnosis. How rare is an adult diagnosis?

Actor Wentworth Miller opened up to fans this week about a surprising medical diagnosis: He has autism spectrum disorder.

Miller, 49, broke down the journey to his diagnosis in a candid Instagram post, featuring a blank white square. “Like everyone, life in quarantine took things from me. But in the quiet/isolation, I found unexpected gifts,” he began. The Law & Order: Special Victims Unit star shared that it’s been nearly a year since he received his "informal autism diagnosis. Preceded by a self-diagnosis. Followed by a formal diagnosis."

“It was a long, flawed process in need of updating. IMO. I'm a middle-aged man. Not a 5-year-old,” he continued. “Let's just say it was a shock. But not a surprise.”

Miller acknowledged that he doesn’t “know enough” about autism to become an advocate. “There's a lot to know,” he said. “Right now my work looks like evolving my understanding. Re-examining five decades of lived experience thru a new lens. That will take time.”

"Meanwhile, I don't want to run the risk of suddenly being a loud, ill-informed voice in the room," he said. "The #autistic community (this I do know) has historically been talked over. Spoken for. I don't wish to do additional harm. Only to raise my hand, say, 'I am here. Have been (w/o realizing it).'"

Miller urged people to look up content on Instagram and TikTok that works to fight stigma surrounding autism, pointing out that these creators have “been schooling me as well.”

“This isn't something I'd change,” Miller said. “No. I get — got —immediately being autistic is central to who I am. To everything I've achieved/articulated.”

He ended on this note: “I also want to say to the many (many) people who consciously or unconsciously gave me that extra bit of grace + space over the years, allowed me to move thru the world in a way that made sense to me whether or not it made sense to them... thank you. And to those who made a different choice... well. People will reveal themselves. Another gift.”

The comments of Miller’s post were filled with message of support. “This was beautiful to read, welcome to the community Wentworth!” one person said. “Thank you for this. Got my autism diagnosis at 30 years old, and it was a peculiar gift, really. Your voice, about this, is so comforting. So, so comforting.” another said.

Miller followed his original post up with another message, saying that he was "awed by the warmth + welcome on this page... and serious warrior energy. Mixed with tenderness. (Gorgeous combination.) Thank you. Proud to be part of it/us/this."

What is autism spectrum disorder?

Autism spectrum disorder (ASD), previously known as just “autism,” is a developmental disorder that can impact a person’s communication and behavior, according to the National Institute of Mental Health (NIMH).

ASD can cause a range of symptoms, NIMH says, including: difficulty with communication and interaction with other people; restricted interests and repetitive behaviors; symptoms that hurt the person’s ability to function properly in school, work and other areas of life.

As for finding out late in life, like Wentworth, "It is common for a diagnosis in adults to take time," Dr. Edward Brodkin, director of the Adult Autism Spectrum Program at Penn Medicine, tells Yahoo Life. "There are many undiagnosed adults, and there's a shortage of clinicians with expertise to diagnose autism in adulthood."

But how common is an adult diagnosis?

Autism spectrum disorder can be diagnosed at any age, but it’s most commonly diagnosed when a person is a child because symptoms usually appear during the first two years of life, NIMH says.

Still, while an ASD diagnosis is more common in children, “Diagnoses are made on adults all the time,” Dr. Danelle Fisher, pediatrician and chair of pediatrics at Providence Saint John’s Health Center in Santa Monica, Calif., tells Yahoo Life.

Many people who are diagnosed with ASD as adults had high-functioning symptoms that were simply missed when they were children, Dr. Thea Gallagher, clinic director at the Center for the Treatment and Study of Anxiety at the University of Pennsylvania's Perlman School of Medicine, tells Yahoo Life. “It happens more often that people will say, ‘You’re fine. You’ve done well in school and you have friends,’” she says. “A lot of people who don’t get the diagnosis until they’re adults assume it would have come out in childhood. It’s really brave to say, ‘I have this hunch about this, and I’m seeking help.’”

ASD needs to be diagnosed by a qualified clinician—“someone in the field of psychology or neurology who knows how to administer the right investigation,” Fisher says. From there, she notes, “it’s important to make sure that an adult realizes that they have autism spectrum disorder, and how they can improve their functioning.”

There’s no one-size-fits-all treatment for ASD, but it may involve medication to help with symptoms like irritability, aggression and anxiety, along with behavioral, psychological, educational or skill-building interventions, NIMH says. Those can include working toward reducing challenging behaviors, increasing or building on your strengths and learning social, communication and language skills.

If you’re diagnosed with ASD as an adult and have started treatment, Gallagher recommends talking to your loved ones about your condition. “You can explain how this has impacted your life, help them understand your diagnosis, and tell them how they can help,” she says. (If you’re not sure, Gallagher recommends talking to your doctor for advice or consulting resources like Autism Speaks’ guide for adults with autism.)

"Everyone on the spectrum is very different, but the conversation with loved ones should ultimately focus on some of the issues you've been grappling with and what has been helpful for you," Brodkin says.

Adds Gallagher, “The first step is understanding what this really means for you. Learning about the spectrum is really important. Having loved ones come alongside, offer support, and aid in the challenges you’re up against is invaluable.”