Built my current machine from the ground up in December 2010. I told myself I wanted everything inside it to be good for the next 3-4 years. I'm surprised with the amount of gaming I do, that I havent had to update my hardware just yet. Next machine might come next year some time, and I'm thinking of going with a ridiculously expensive build that should last me 5 years.
My mac is 2011 – I only use it for graphics and video editing, so until it starts slowing down rendering large files I can get by with what I have. The desktop pc is one I built myself, and parts have come and gone, so putting an age on it is tough other than to say that lots of it dates back to late 2009. One laptop is a 2008 (!) – it was a custom build at the time and still even handles most games surprisingly well - and the other machine is an off-the-shelf 2012 that I spent way too much on but it runs everything I could want it to smoothly.