10.8.5 won't include the kexts needed to support the hardware in the 2014 MBA. The TB2 ports won't work, graphics may be an issue (different graphics processor than any model supported by 10.8.5 would have), among other things. This is the reason Apple says you can't use an OS that is older than that which the computer shipped with.
Exceptions to that would be like a Mid 2012 MBP that was bought in December, it could still run Lion even though it shipped with ML since the model was originally shipped with Lion in June 2012.