Upgrade macbook air late 2008 to mavericks not working

I have a late 2008 macbook air and im trying to install mavericks, when i press download and install on app store it says it cannot be installed on that macbook air. but according to the specs on the apple webpage its compatible with mavericks. im currently running snow leopard. I tried to make a bootable disk using disc utilities, everything is done correctly and i start up my macbook air holding option key and choose my bootable USB as startup disc, but i just get a circle with a line through almost like a prohibited sign. What must i do?