Here's a nice chart I made to show the fragmentation of desktop operating systems - so I'm combining what the current market share is for each O.S, against when it was released. Basically what we are seeing here is Windows users are running some of the oldest operating systems available, whereas the Mac creates a trendline.
Information for Linux isn't available, but generally they are patched very fast, and account for a very small amount of desktop computers on the internet. I've explicitly not included other operating systems older than 2000 because they are less than 0.01% usage share.
Raw data source:
Net Market Share
