Cognitive decline as early as 18 years prior to clinical diagnosis of dementia
Performance on individual cognitive tests of episodic memory, executive function, and global cognition also significantly predicted the development of AD dementia, with associations exhibiting a similar trend over 18 years.
Conclusions: Our findings suggest that cognitive impairment may manifest in the preclinical phase of AD dementia substantially earlier than previously established.
http://www.neurology.org/content/85/10/898 (paywall site)