From CBS News:

A 2006 study questioned the acceptability of minor episodic memory loss in older adults as normal. Episodic memory loss includes things such as forgetting the name of a new acquaintance, a recent conversation, or an upcoming appointment.

People commonly undergo an age-related slowing of the ability to retrieve information. They might forget where they put their keys, but they usually remember eventually. But when Alzheimer's is involved, new information is never properly stored, meaning the affected person never learned it well enough to be able to retrieve it.

Related:
Senior Moments: Signs Of Alzheimer's? CBS News.
Memory problems not a normal sign of aging: study http://goo.gl/xb9M