a) The age of a document
b) The age of links leading to and from that document

According to Google,

Stale content refers to documents that have not been updated for a period of time and, thus, contain stale data (documents that are "no longer updated, diminished in importance, superceded by another document").

The staleness of a document may be based on:

a) document creation date,
b) anchor growth, traffic,
c) content change,
d) forward/back link growth, etc.

According to Google patent, there are 4 factors that determine the "stale" factor of a document:

Query-based factor: are those factors that analyze as to which pages in SERPs are selected by users.

Link-based factor: analyzes the page backlinks.

Traffic - based criteria: If there is a large reduction in traffic, then it may mean that the content or the document is stale.

User-behavior-based criteria: if people spend too little time on the page, then chances are that the contents are not fresh.