And vintage hardware will not support newer OSX upgrades
I have an original MB Air that is now stuck at 10.7.5
Mrs. Fierro, too.
She doesn't know it yet but she's getting a maxed-out 13" A1425 Macbook Pro for our wedding anniversary in a few weeks.
Unfortunately that is true. Good thing that's a pretty good OS.
On the other hand, the current retail value of the MacBook Air is $550 to $650 and it might behoove you to sell it an upgrade to a brand new one which you can buy for just $899. . . and get a Hollywood reboot like they did for Star Trek. You can also get a Refurbished 2014 model for (Oh, MY! I just looked!) $679.00 which has 1.4GHz Dual-core Intel Core i5 with 4GB of RAM and a 128GB SSD which will probably beat that old one all hollow, and comes with OS X.10 Yosemite and a brand new warranty.
Heck, with tax and all after selling your old one on eBay, that might only cost you around $100. You can't beat that with your gavel!