In off-Broadway’s “A Life,” David Hyde Pierce’s 54-year-old character realizes he’s forgotten what he was talking about.

That’s no big deal, says Pierce, 57: “It’s the kind of forgetting we all experience at this age.” What isn’t normal, he says, is the massive memory loss that accompanies dementia, especially Alzheimer’s disease, an illness with which he’s sadly familiar.

The nation’s sixth-leading cause of death, Alzheimer’s afflicts 5.4 million Americans and affects the lives of 15 million family members and friends who are their caretakers.

Finding the funding to fight it is something the “Frasier” star has been doing since 1994, when he played TV’s “Celebrity Jeopardy” and picked a cause to which to donate his winnings. He chose Alzheimer’s, having seen the toll it took on his grandfather.

“We come from a family where you’re not supposed to have any problems, and if you do, no one’s supposed to know,” says Pierce, who grew up in Saratoga Springs, NY. “Dementia and Alzheimer’s can last a long time. It’s very difficult to care for someone who no longer recognizes you.”

Pierce is a national spokesman for the Alzheimer’s Association, testifying before Congress on the need to support research.

“My grandfather’s personality really changed,” Pierce tells The Post. “He went from not being able to play chess anymore to being strapped into the armrests of a wheelchair in a nursing home.”

‘Dementia and Alzheimer’s can last a long time. It’s very difficult to care for someone who no longer recognizes you.’ - David Hyde Pierce on his grandfather

His grandfather’s memory began failing in the late ’80s. It wasn’t until a few years later, when the extended family dined at a restaurant, that they realized something was terribly amiss.

“He was looking at the menu, and it was clear that he couldn’t read it,” Pierce says of his grandfather, then in his 80s. But there were other signs: Always a gentleman, he was now prone to anger. When he knocked his wife to the ground, she knew she could no longer care for him.

A different kind of dementia struck Pierce’s father, George, an insurance salesman and amateur actor, after he suffered a stroke in the early ’90s following heart surgery. Pierce’s mother handled their finances until she died in 1995. Pierce believes stress wore her down. His dad died three years later.

Pierce is cautiously optimistic about the recent discovery of an antibody that could destroy the brain plaques that cause Alzheimer’s, though genetic tests are still inconclusive. “If you have [Alzheimer’s] in your family, it doesn’t mean you’ll get it,” Pierce says, “but for every person in your family who has it, your chances do go up.”

He stresses the importance of diet and exericse. “All the things you should do to keep your heart healthy,” he says, “will keep your brain healthy.”

David Hyde Pierce will be honored Nov. 7 by the Drama League.