WASHINGTON — Ron Reagan’s new memoir, “My Father at 100,” has touched off sensational headlines with its suggestion that President Ronald Reagan might have begun showing hints of Alzheimer’s disease while still in the White House.

But in two interviews this month, the younger Mr. Reagan said he never meant to suggest that his father had dementia before leaving office in 1989. And he graciously took the blame for not being more explicit in a passage that described a few personal observations along with comments from the former president’s doctors.

A “rather small section of the book has attracted outsize attention,” he said in a telephone interview from Seattle, where he lives....