I've sold all of my Macs (in the past 12 years, anyway) on eBay. I had two 2006-vintage models - the 'original' Intel 2006 iMac and the original Intel 2006 MacBook Pro. I sold my iMac when I moved into a nursing home in mid-2006, so I didn't have it long, but I kept my 2006 MacBook Pro until March of 2012 - a full 6 years - when I purchased the current late 2011 15" MBP that I have now (and finally got out of the nursing home!).
I've had Mac's since 1985 and my usual 'recycle time' has been about 3 years, sometimes, more sometimes less. My rule of thumb, now, is that I 'trade up' every three years - shortly before or after my AppelCare extended warranty runs out. It's a good rule of thumb, I think, and it allows me to put enough money aside each month in savings for my "new" future Mac.
I think that 5 years is a good run - and, if it's still working for you I wouldn't worry about not being able to upgrade to the latest OS. There are software issues to consider, too, when you upgrade both your machine and your OS - much of the software that works now on your machine might just simply not run on OS X Mountain Lion. I spent more updating my software when I bought my new MacBook Pro than I did on the machine itself - so you might want to count yourself lucky that you can't upgrade.
Technology changes - and it costs you (quite a lot, sometimes) to stay up-to-date with the latest hardware and software. It's not built-in obsolescence, really, but just the speed with which technology moves. Is it time for you to get a new Mac? You'll have to be the judge of that - keeping in mind that you may have to upgrade a lot of software in the process...
Good luck,
Clinton