What are some of the stages of memory decline?

Decline varies. I'd like to ask what you mean by "decline." the first thing that comes to mind for me is dementia, which tends to have a gradual onset and progressive decline. If this is what you're asking about, the symptoms vary across people. The brain itself develops physical pathology associated with multiple cognitive deficits, especially worsening memory problems. Other cognitive problems occur as well.
Continuum. Memory failure is usually graded as part of an overal picture of mental deterioration which usually starts as a subclinical (hard to diagnose) cognitive impairment, progressive to mild to moderate to severe cognitive impairment to dementia.