Diamond Age (also known as The Glitter Age, formerly believed to be The The Ruby Age) is the inevitable upcoming post-Moffat/Capaldi era that will begin in 2018, when Chib Chib McNips will officially take over as showruiner beginning with Series Dubs.
The Beginning of the End Edit
While (obviously) little is known about the nature of the Diamond Age at this point, the general consensus among /who/ is that this signals an unstoppable decline in quality that spells the end of Doctor Who as we know it, and perhaps even the end of the show itself.
Rumors are flying that Capaldi will bow out once The Moff leaves (parallel with Tennant and RTD in 2009-10). If this is true, it spells out what /who/ has collectively deemed the Bad Wolf Nightmare Scenario and eventually leads to the inevitable 2018 cancellation.
Change, my dear Edit
Of course, one should note that Doctor Who is a show all about change, while it's fans (especially /who/res) are all about things staying the same. Oh, irony. You're one bitter mistress.