What causes Alzheimer's to progress quickly? Could it be a different dementia?

My uncle was diagnosed with Alzheimer's in May of 2009 and he is already at stage 6 with frequent delusions and wandering. Is it normal for Alzheimer's to progress so quickly, or could he have another type of dementia that has gone undiagnosed? He is only 62 years old. There has only been 1 other person in our family with Alzheimer's, my great uncle, whose progression took almost 10 years to reach the stage my uncle is at now.