From my understanding spectrometers (Colormunki Design and i1Pro) do not really degrade with age. These types of devices read actual waves of light and do not use color filters. They might get off somewhat with time but to a much lesser degree than colorometers. In other words, unless you are a professional, you will probably be looking to buy a new spectrometer before it is off enough to really matter.
Colorometers (just about everything else) uses filters which degrade over a relatively short period of time, especially the red filter. Imagine the filter slowly turning lighter or fading, that is what keeps these instruments from being useful long term.
The fix is to get both types, and use the spectrometer to profile the colorometer. In effect, you are reading the primary colors with each device, and then using the data to figure out how much the color filters are faded in the colorometer. Using this information you can adjust the readings from the colorometer to show the correct values even with the degraded filters. Of course these complex calculations are usually done within the software you are using to calibrate with. So you do not need to worry about the calculations yourself.
So now you might ask why to use a colorometer at all when you can just use a spectrometer. The reason is that colorometers are generally faster than spectrometers and spectrometers (at least consumer level devices) have problems reading low light levels on the grayscale effectively. Therefore, a properly calibrated colorometer is actually more accurate under low light levels than a consumer level spectrometer.
Of course, I am new at this, too; so hopefully an expert will come along and verify what I have just said.