A shockingly frail NANCY REAGAN is slipping mentally, and friends fear the former first lady is in the early stages of Alzheimer’s disease – the same killer that claimed her husband’s life.

During a recent outing, the 89-year-old widow of former president Ronald Reagan was stooped over and so shaky that she clung to her Secret Service escort for support.

But insiders say her mental decline is even more pronounced than her physical deterioration.

"Nancy will sometimes sit for hours in a chair at her home in Bel-Air with a distant look in her eyes," confided a longtime pal.

"She’s also started to become very forgetful. She can be very lucid one moment and then completely lose her train of thought the next. The fear is that she’s in the early stages of Alzheimer’s or senile dementia."

Nancy perks up only when someone mentions her late husband, who passed away at age 93 in June 2004, said the friend.

"Sometimes, Nancy gets all done up at a hair salon in Beverly Hills as if she were getting ready for a date with Ronnie," the source revealed.

"In Nancy’s mind, he’s still around. She talks with him all the time."

In a 2009 interview, she admitted, "It sounds strange, but…I see Ronnie. At nighttime, if I wake up, I think Ronnie’s there, and I start to talk to him. It’s not important what I say. But the fact is, I do think he’s there. And I see him."

The former first lady has fallen and has been taken to the hospital three times since 2005, but despite her fragile health, she greeted a crowd of dignitaries at a celebration of her late husband’s 100th birthday on Feb. 6 at his presidential library in Simi Valley, Calif.

"I know that Ronnie would be thrilled," she said, then looking heavenward, added: "and is thrilled to have all of you share in his 100th birthday."

But Nancy looked so fragile at the Beverly Hills restaurant Spago on Feb. 28 that the Internet buzzed with concern.

"Poor woman. She looks so frail," wrote one person, while another posted this message: "Senility and fear of death is a terrible thing."

Added her pal: "The only thing Nancy lives for is Ronnie – just as when he was alive.

"She said, ‘I miss him so much! But I feel him here with me, and I honestly believe that we will be united again very soon – in heaven!’"