Which Mac Mini is the earliest able to run Yosemite?
See http://www.apple.com/osx/how-to-upgrade/:

These Mac models are compatible with OS X Yosemite:

iMac (Mid 2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)