As a rule of thumb, it's from one to four generations preceding the current generation of video games, when video games is interpreted to mean "console video games."

This is due to the fact that this is the time required for a genre (2-D shooters, for example, or graphic adventures) to die out or become extremely marginalized, as well as the time required for some major revolution to pass, causing the long-time fans of those genres marginalized or revolutionized to feel nostalgic about the way things were.

Suggested Golden Ages of video games include:

Some periods that are never referred to as the "Golden Age" of video games:

In short, the "Golden Age" of video games can be assumed to be whenever the speaker started playing video games. In addition, the speaker will generally comment that after the epoch he or she refers to as the "Golden Age" was when games stopped being about creativity or fun and started being about money or commercialism or somesuch.

This effect can be attributed to the fact that games from said "golden age" that were pretty but not fun, or alternately obvious attempts to bilk unsuspecting gamers generally didn't get remembered, unless they were especially infamous.