This is a time where everyone always did the right thing
so you didn't have to consider many process and people
failure scenarios.

Development groups are social institutions and as such will
behave in the same way society behaves. In other settings
we have judges, courts, policemen, locks on our doors...
in general an expectation that people will not perform
flawlessly or behave as expected. It's naive to assume people will
ReFactor, create and maintain great UnitTest''''s, and do
many of the other things ExtremeProgramming expects of people. The smaller
the group the more likely the GoldenAgeOfMan is true. But
if you get into large groups of people with different lives,
agendas, etc, the GoldenAgeOfMan can not be assumed.

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A "golden age" group could be caused by having either extraordinary people who just happen to do all the right things right, OR by having people who don't have to be extraordinary, but who are diligently following an extraordinary process such as XP.  A defined, written, teachable, and followed process at a high CapabilityMaturityModel level always reduces the variability of the results.  Learning and following the process is fairly easy and fun as well as being highly effective and productive.  It is designed to take into account the various agendas of the various participants, formalize an explicit negotiation to shared expectations, and increase the likelihood of delivering on those expectations.  It's naive to assume that developers will spontaneously start and continue to refactor well, use unit tests well, and so forth, that's why XP is written down so that developers and others can learn these skills and put them into practice. From the site you can see that XP failures are primarily related to stakeholders who, for emotional reasons unrelated to the project's success, declined to participate in the negotiation of clear and shared expectations.
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In CliffordSimak'''''''s '''City''', man is long extinct and uplifted Dogs have inherited the Earth, sitting around campfires discussing the GoldenAgeOfMan and whether it is all just myth and legend.

I'm unsure if this is related to the above meaning or not. ;-)