Glen Campbell downplays Alzheimer's, asks "When's it gonna get here?"

(CBS) "I ain't felt anything yet - when's it gonna get here?" That's Glen Campbell talking about Alzheimer's disease. The country music icon, who has acknowledged that he has the devastating neurological disease, said in an interview with ABC News correspondent Terry Moran that the symptoms hadn't been especially noticeable, adding, "I've always been forgetful."

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But moments later Campbell had to ask his wife, Kim Woollen, how old he was. He guessed 78. "Seventy-five," she corrected him.

Campbell, whose hits include "Rhinestone Cowboy" and "Gentle on My Mind," got the devastating diagnosis seven months ago.

And it is devastating. Alzheimer's slowly destroys memory and cognition and eventually makes it impossible to carry out the simplest tasks of everyday life. Doctors don't know the precise causes of Alzheimer's, which affects an estimated 5 million Americans, but suspect that heredity and "lifestyle factors" play a role. These include high blood pressure, diabetes, obesity, and other chronic conditions.

Four prescription medications have been approved for the treatment of Alzheimer's. But while the drugs can help maintain mental function and curb behavioral problems for a few months to a few years, they do nothing to change the underlying disease process.

How long does Campbell have? No one knows. But the Alzheimer's Association says that people with Alzheimer's live eight years, on average, after their symptoms become noticeable to others.

But Campbell isn't laying down his guitar just yet. In fact, he's releasing a new album and is planning a farewell tour.

"I have been blessed," he said. "I really have."