What is the oldest version of Mac OS that can be used on retina displays? For development (backward testing with older OS versions) I need to install older mac OS versions.  Ideally I'd want to go as far back as Snow Leopard.

I have an iMac with retina (model  iMac15,1).  Before I hunt and dig around for old installation media and experiment, I'd like to find out what is the oldest version that properly supports retina displays (i.e. allows for a properly readable screen)?

The web says Retina was introduced with Macbook Pro 2012 which would be around Mountain Lion.

Expierences (first hand) anyone?
The limitiation isn't so much the display as the hardware overall. You can't install OS versions older than the hardware because older versions don't bring all the required drivers etc. for various components.

Your iMac (late 2014) came with OSX 10.10 pre-installed, so this is as far as you can go back.