At some point you can't keep upgrading operating systems. I have an iMac which had to be left behind this time around.
Make sure your Mac can run Mountain Lion.
Your Mac must be one of the following models:
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)
If you are running Lion, you can find out if your current Mac qualifies by clicking the Apple icon at the top left of your screen, choosing About This Mac, then clicking More Info.