Mountain Lion OS X Installation on 2009 Macbook

I have a 2009 Macbook, fully updated and running OS 10.6.8. I tried installing Mountain Lion, but keep getting the message that it is not compatible with this computer. It has a new hard drive and has been verified through Disk Utility. There is not Time Machine Backup set up for it or other kind of recovery drive. I'm not sure why it won't install, I didn't have any issue with my 2011 Macbook Pro or my 2010 iMac. Is the 2009 system too old for Mountain Lion?