Yes, beause it's too old. It's missing some hardware that Mountain Lion wants to run efficiently.
These are the supported Macs for Mountain Lion:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- Xserve (Early 2009)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
That doesn't matter. It's not about when you purchased it... Apple is referring to the "model designation" which talks about the physical hardware that makes up the machine.
If you bought that exact same machine from Apple today, it still wouldn't run Mountain Lion. The technology inside of it (chipsets, processors, motherboard, EFI, etc.) are all 6+ year old technology.
Errr.... it's not a matter of when you bought your Mac, it has to do with your Mac's hardware specifications. See William Llyod's post.
I feel your pain. I have two outdated MacBooks. From what I can tell, it's the GPU that's holding them back. Mountain Lion relies more heavily on them and so those Macs sporting older GPUs got the ax this time around.
Seriously. Your machine is 6 years old.
Here's the thing: Hardware is always improving. It's not just getting faster, but it's also getting new functionality. The problem is, if you code to the "lowest common denominator" then your software (or operating system) isn't as fast, efficient, and in general "quality" as if you leverage the new features that the hardware offers.
At some point, it makes sense to leverage new hardware. 6 years is a pretty good run, and your machine can still run Lion perfectly well.
To expect to buy a computer and have it be able to run the latest and greatest OS for say 10 years is simply an unrealistic expectation. End of story.