Amyloid -- an abnormal protein that's a hallmark of Alzheimer's -- starts accumulating inside neurons of people as young as 20, reports a study. This is the first time amyloid accumulation has been shown in such young human brains. Small toxic amyloid clumps were found in neurons of deceased young adults. The clumps grew larger in the brains of normal older adults and those with Alzheimer's. The clumps likely damage and eventually kill memory-related neurons.