The problem is the OS rev:
- The 2006 mini became obsolete 4 years later in 2010 when OS rev 10.7 (lion) was introduced.
- The 2007 mini became obsolete 5 years later in 2012 when OS rev 10.8 (mountain lion) was introduced.
- The early 2009 mini is still supported 6 years later in 2015 when OS rev 10.11 (el capitan) is introduced.
Obviously the 2009 minis (and probably the 2010 as well) are the next to go.
Obviously but "when" is the question. Note that they are actually supporting longer now than they used to. The 2006 mini became obsolete because of the 32-bit architecture. I don't know what the issue with the 2007 is, maybe its awful embedded graphics. There is no move to 128-bit architectures in the works so I'd expect that as long as they keep the graphics drivers updated all is good. Note that even though you can run El Capitan on a 2009 mini, and it works fine, not all features of the latest OS are supported. You could call this a graceful dropping of support.
Moves in the recent OS versions to reduce the memory footprint, increase performance and energy efficiency will actually increase the useful life of older machines.