The signs were subtle at first. Then obvious. Then terrifying and heartbreaking all at once.

For Julie Karceski, it was when her mother had small memory lapses and became angry if anyone pointed them out.

For Dylan Cooke, it was when her mother forgot to pick her up at the airport after Dylan’s return from a year in Nicaragua.

For Lindsay O’Bannon, it was when her mother couldn’t seem to master the simple rules of the board game her family was playing at a holiday gathering.

Each of these three women knew that something was very wrong with her mother. Yet, until doctors pronounced the diagnosis as early Alzheimer’s disease, dementia seemed out of the question. Their mothers were just too young — and so were they, these adult children whose lives changed profoundly just as, in some ways, their lives were about to begin.

Dylan Cooke is shown with her mother, Rae Lyn Burke, a scientist who was diagnosed with early-onset Alzheimer’s. (Family photo)

“Suddenly, I was facing a life path I really hadn’t considered before,” Cooke said, recalling the day her mother received the diagnosis of early dementia. “I was facing losing my mom to a pretty horrible disease. I don’t think people are ever ready for something like that, but I certainly wasn’t ready for that, at 24, that day.”

Early-onset Alzheimer’s is particularly cruel. The disease strikes some in their prime, when careers are at their height and the rewards of a lifetime’s work at last seem within grasp. It destroys memories and the ability to learn, then wipes away most of one’s identity, leading eventually to death.

Young adult children also bear a terrible burden. Twentysomethings — those who only recently tasted independence and those who still lean on their parents from time to time — can find themselves thrown into the unfamiliar role of their parent’s caregiver. Some put their lives on hold, postponing graduate school or marriage. Others pass up job opportunities that would take them away from home. Just as often, they feel pressure to get on with major milestones, to accomplish the big things in life before their stricken parent departs.

Even support groups — whose members are sometimes as old as their stricken parents — can be an awkward fit. And for some adult children, a parent with early-onset Alzheimer’s means that their inheritance might include a gene that predisposes them to early dementia. That’s the case in “Still Alice,” the acclaimed independent movie starring Julianne Moore that has drawn attention to early-onset Alzheimer’s and the destruction it visits on families.

“You just find yourself in this suspended state for years because you don’t know how to move forward,” Karceski said. Karceski said her mother, Dawn, who was a registered nurse, was in her 50s when she began showing symptoms of dementia. Julie Karceski was then an undergraduate. With her mother sick, Karceski felt guilty about continuing with her schooling and put it off for several years.

“I know I’ve heard other people say that they’ve received family pressure to marry while their parents are still alive, which is just silly,” said Karceski, who is a graduate student at MIT. Her mother, 61, is in a facility for people with dementia and has trouble recognizing family members.

Early-onset Alzheimer’s is, thankfully, the rarest form of the disease. More than 5 million Americans are living with the disease, most of whom are 65 or older, according to the nonprofit Alzheimer’s Association. Early-onset Alzheimer’s affects about 200,000 Americans who become symptomatic before the age of 65, said Maria C. Carrillo, the association’s chief science officer.

Some have Dominantly Inherited or Familial Alzheimer’s Disease, a form of the dementia-causing illness that accounts for about 2 percent of all early-onset Alzheimer’s cases, Carrillo said. This form is passed on by gene mutations that cause an increase in beta amyloid, whose buildup sometimes triggers a series of other pathological changes in the brain. A child whose father or mother is carrying the abnormal gene has a 50 percent chance of inheriting it. If it’s passed on, the child will almost certainly develop dementia, Carrillo said.

Moore, who has been nominated for an Academy Award for best actress for “Still Alice,” modeled her performance in part on the experiences of Sandy Oltz. Oltz, a former surgical nurse from Sartell, Minn., was diagnosed with Alzheimer’s in her 40s. Oltz, who is now 50, said physicians first thought her memory and cognition lapses were caused by stress.

“I was putting things away in the wrong places. I was forgetting to pick up my son from baseball practice. It was just not typically me,” Oltz said. State-of-the-art diagnostic testing at the Mayo Clinic revealed early-onset Alzheimer’s.

“I was in shock at first,” she said in a telephone interview last week. “I was kind of relieved to have a name [for the symptoms]. At first, I thought I was going nuts. But to have that name was not the name that I wanted.”

Oltz reviewed the script of “Still Alice,” which is based on Lisa Genova’s novel, and talked by Skype or texted with Moore almost daily, Oltz said. Moore pressed Oltz to describe what it felt like to have Alzheimer’s, or what was it like to go for a run and become lost. Her experiences show up in the movie in a startlingly real way, she said.

“It was just surreal,” Oltz said. “It was me. And she’s playing a 50-year-old and she’s a redhead, and people say we look like sisters. Kind of spooky.”

Cooke, now 31, also served as a consultant to the movie. She helped actress Kristen Stewart understand what a young adult goes through when her parent develops early-onset dementia.

Cooke’s mother, Rae Lyn Burke, not unlike the movie’s title character, is a well-known scientist who helped develop an Alzheimer’s drug that she is now taking to slow the disease’s progress. Cooke was with her when her mother was diagnosed at the age of 60 with Alzheimer’s.

“What do we do now?” Cooke recalled asking after they left the doctor’s office.

“Well, I’m going to work,” her mother replied.

But Cooke reacted differently.

“I totally fell apart. I got on the phone with my best friend and sobbed,” Cooke said. Since then, Cooke said, her life has been a lesson in accepting what she can control and what is beyond anyone’s control. And she struggled to balance her career with her duties as a caregiver.

“I’ve spent a lot more time than I would otherwise with my mom. I’ve had to face parent mortality and caregiving at an age when a lot of my peers weren’t doing this,” Cooke said.

O’Bannon, 29, who grew up in Warrenton, Va., said that in the early stages of her mother’s disease, their relationship became difficult for reasons neither understood at first. Her mother, Lisa, an award-winning kindergarten teacher, had begun forgetting things and seemed to be under too much stress. She forgot her daughter’s birthday and engaged in conversations about topics as if they had never discussed them before, although they had.

“Are you on drugs, mom?” O’Bannon recalls asking her mother. “You don’t remember the conversation we had yesterday?” She said she began to wonder if her mother cared about her.

Then over Thanksgiving one year, her mother could not follow the rules of a board game.

“We were all making looks and realizing how weird it was,” O’Bannon recalled. “And she was kind of oblivious.”

Following her mother’s diagnosis, O’Bannon tried to cope with her new role as caregiver.

“Your 20s are insane — trying to figure out where you’re going, what you’re doing — and I didn’t have a mom to talk to. I became a mom,” O’Bannon said, referring to her newfound role as a caregiver.

Her uncle shouldered much of the day-to-day responsibility for O’Bannon’s mother, while O’Bannon struggled to find her balance, poised between the call of life in the District with other young people and the call of duty back home.

“I would stop going out with friends because you’d be surrounded by people who had no idea what was going on in my head. It was so unimaginable you almost felt weird telling someone this story about what you were going through,” O’Bannon said. She said it helped joining a support group at Iona’s Adult Day Health, Wellness and Arts center with an unusual number of young caregivers.

Tyler Summitt was 20 years old when he found out that his mother had early-onset Alzheimer’s. As the only child of divorced parents, he began taking on the duties of caregiver on the day after her diagnosis. He arranged for his mother’s local doctors to speak with physicians at the Mayo Clinic, where the diagnosis had been made. He called the family’s lawyer. He called their financial adviser.

He helped his mother strategize about how to manage the news, because his mother was also a public figure: Pat Summitt, the indomitable women’s basketball coach at the University of Tennessee. Over 38 years with the Lady Volunteers, she became the NCAA’s all-time winningest coach in men’s and women’s teams.

“You kind of go from being taken care of to taking care of your parent,” said Summitt, who is head coach of Louisiana Tech’s women’s basketball team. His girlfriend at the time — AnDe, who is now his wife — pitched in, as did a deep network of friends and staff.

His mother, who stepped down as coach in 2012, still keeps up a healthy schedule of physical and mental training, and she remains an active and enthusiastic booster of the team, Tyler Summitt said.

“She’s sharp. She jokes, ‘I thought I retired. But I don’t think I did because of how much I do,’ ” he said. Eventually, they started the Pat Summitt Foundation.

Looking back, Summitt said the initial shock after his mother’s diagnosis — and maybe the upbringing of a woman who exuded mental toughness — helped him take action without focusing on the emotional toll of the news.

“It was weird,” he said. “It was almost as if she had trained me to take over. When I look back, I don’t recall any emotion. There was so much to do, there was almost no time to think about it. I just had to be strong. I just had to spring into action.”