Quoted from CNET (
here):
"… if you haven't updated your Mac's operating system in a number of years, then you need to check to see if you are running at least OS X 10.6.8 Snow Leopard, which was released way back in 2009. Its 10.6.6 update introduced the Mac App Store, which you'll need in order to download El Capitan. You need be running one of the following:
- OS X Snow Leopard (10.6.8)
- OS X Lion (10.7)
- OS X Mountain Lion (10.8)
- OS X Mavericks (10.9)
- OS X Yosemite (10.10)
If you have an ancient Mac with an OS predating Snow Leopard, you will need to install Snow Leopard before then moving to Yosemite."
HTH
Edit - think that last sentence is a typo, in that you should be able to go from Snow Leopard > El Capitan, rather than needing to install Yosemite.
I am still running Lion, so hopefully I can go straight to El Capitan as I do have access to the Mac App Store on my machine currently (which seems to be the prerequisite)