Hey there Kindred - great question you have - one that seems to draw different conclusions. Some say to drink it within a few years, some say in less than 15. It's not that it goes 'bad' so to say, it just may lose flavor if air seeps into the bottle or if it's exposed to extremes of temperatures.
My suggestion to you is to always keep it tightly sealed, keep it in a cabinet so it's not exposed to light and also keep it in the presentation box. That might be a bit too much keeping it in the box, but what's the harm? I'm not so sure it's necessary to keep it in the fridge - just keep it in a cool, dark place. I have an 18 year Glenlivet which was purchased more than 6 years ago and I have not noticed any change of taste.
As for keeping it in a decanter - the chances of air seeping in are so much greater. So while I'm sure it's a great presentation, as for me, I would not keep my Scotch in a decanter for a long period of time.
Enjoy and bottoms up!