Would the personal memoirs of an individual written decades after an event be considered a primary source for that event? How do historians view such sources?
The primary/secondary source distinction refers to how an item is being used and is not some sort of intrinsic characteristic.
Primary sources are the subject of study, while secondary sources are discussions around the subject of study.
For example, if I wanted to research "reactions to the First Crusade", my primary sources would be literature discussing the First Crusade and my secondary sources would be any literature discussing the primary sources. If instead I want to research modern perceptions of the reaction to the First Crusade, what were originally my secondary sources would become my primary ones.