Cannot upgrade to Maverick

What does the message say exactly?

At the top of this page is a link that says, Learn about OS X Mavericks System Requirements. I suspect that although you may have purchased it in late 2008 it doesn’t actually mean it’s a late 2008 model. Your Mac is probably too old.

If you go>

Utiltiies> System Profiler> Hardware

You’ll see an entry next to Model Identifier, what is it?

After you find the Model Identifier, go to this page:

http://support.apple.com/specs/

Copy the Serial number in the "Search Tech Specs" search box. This will take you to the info on your computer. You can also check to see how much RAM you can install.

While 2GB RAM is the minimum recommended, I suggest you upgrade your RAM to 4GB or higher. Otherwise you might be better off in Snow Leopard.

Check that your computer is compatible with Mountain Lion/Mavericks.

To check the model number hold down the option/alt key, go to the Apple menu and select System Information.

MacBook Air (Late 2008 or newer) model number 2,1 or higher

Sorry, but the MacBook Air1,1 is limited to OS X Lion. The model was introduced in January 2008, so it's not called a Late 2008 MacBook Air.

The dates used are for model introduction, not dates of purchase. The Late 2008 MacBook Air2,1 was introduced in October 2008.

While 2GB RAM is the minimum recommended, I suggest you upgrade your RAM to 4GB or higher. Otherwise you might be better off in Snow Leopard.

MacBook Air RAM is not upgradable.