Ref:It seems DirectX 12 will only be available to those running Windows 10 and Windows 8.1, so if the API lives up to its name, and game developers make full use of it, this may finally be reason enough for gamers to upgrade from Windows 7.
http://www.winbeta.org/news/microsofts-directx-12-api-set-will-reportedly-not-be-available-windows-7
The good news is the cards like Nvidia 600 series and AMD's 7000 series can run DX12 so it won't be a case of upgrading although I've heard this before with previous DX releases and it seems the devs always manage to find a way of making it exclusive to a series of card. I hope this time it's the other way round.