Broadly speaking, it's unwise to fall too far behind the latest when it comes to the OS. You don't at all have to upgrade to El Capitan this minute or anything, but you don't want to be still running something four versions back. There's no security fixes issued for OS versions past that, and that creates risks, so as a rule of thumb you want to be running the current OS version (ideally), or the most recent version, or maybe the one before that, but no further back than that.
If your machine CAN'T run the latest OS version, it's time to look at upgrades or a new(er) machine, even if you're not planning to upgrade. It's Apple's way of telling you that you are getting close to an unsupported system, and as I said that's a security risk.