AirPlay
I have an iMac which is about 4 years old, it has Mavericks installed yet i don't seem to be able to get AurPlay. is this due to the age of the Mac?
thanks.
iMac (21.5-inch Mid 2010), iOS 7.1.2
Sorry, but it looks like your iMac didn't make the cut, i'm not entirely sure but only macs with integrated intel graphics work with airplay.
About AirPlay Mirroring in OS X
If you really are interested there is always airparrot - http://www.airsquirrels.com/airparrot/ which tends to work alright.
If not then you can always just go the good old way of just hooking it up with a cable!
Good Luck!