AP MEDICAL WRITER WASHINGTON -- Scientists have discovered molecular janitors that clear away a sticky gunk blamed for Alzheimer's disease - until they get old and quit sweeping up. The finding helps explain why Alzheimer's is a disease of aging. More importantly, it suggests a new weapon: drugs that give nature's cleanup crews a boost. "It's a whole new way of thinking in the Alzheimer's field," said Dr. Andrew Dillin, a biologist at California's Salk Institute for Biological Studies who led the new research. The discovery, published Thursday by the journal Science, was made in a tiny roundworm called C....