Forgetfulness, long considered a symptom of aging, might really be a sign of Alzheimer’s disease, a new study says.

“If Grandpa is getting forgetful, it’s not from age. There’s something wrong with his brain,” said Dr. David Bennett of Rush University Medical Center, in the Chicago Sun-Times Tuesday. He is the lead author of a study in the journal Neurology.

Researchers examined the brains of 180 elderly nuns, priests and brothers who volunteered to undergo annual cognitive tests and to donate their brains after their deaths. Three-fourths of the 37 volunteers who showed mild cognitive impairment suffered brain damage from Alzheimer’s or stroke. Less than one-fourth had no evidence of disease.

While early research found that only one in 10 people over 65 have Alzheimer’s, the new study could raise that number since one in four elderly Americans have cognitive impairment, and many likely suffer from Alzheimer’s.