What properties help mead to age without skunking? I have done a little home-brewing (with friends) and have made or helped make several meads over the years -- enough to prompt this question but not enough to provide sufficient data to answer it.

Some of our meads have kept well for several years.  Others have started to go off after six months or a year (but were fine before that).  The meads are bottled in capped (usually) 12oz brown bottles stored in a dark basement, and it doesn't seem to be about individual bottles.  This seems to be a case of "some recipes keep longer than others".  I'd like to know what properties contribute to this.

So far, the batch that has kept well for the longest (over 10 years now! I've been saving the last few bottles...) was very high-gravity for a mead; it's strong, sweet, and very smooth.  But another moderately-high-gravity mead degraded after a year or so, and lower-gravity ones have sometimes lasted longer.  So it seems like there must be other factors at play; it doesn't seem to be as simple as "stronger (or sweeter) meads keep longer".

Our production techniques have been about as consistent as home-brewing can be -- same equipment, same general process, same cellars, same fastidiousness about sanitation.  Varieties of honey and yeast have varied, and of course recipes have too.

This is not a home-brewing question; I'm not asking specifically how to make long-lived mead.  Rather, I'm asking what characteristics allow a mead to age longer without going off, which also helps me decide what to drink.

I know that commercial meads exist, by the way, but I have never encountered one.
One of the biggest determinants in how well a mead or beer or wine will age is oxidation. The only time that oxygen is beneficial is when your must or wort (IDK the currect term for unfermented mead) has cooled and you are ready to pitch the yeast. Oxygen at this point will help the yeast to multiply. But when the liquid is hot prior to cooling, and after fermentation is complete, you should be careful to minimize the amount of oxygen you are adding to it. Always stir gently and avoid splashing when racking and bottling. 

You use the word "skunking" in the title. True skunking (as opposed to staling or oxidation) is caused by a chemical reaction between hop extract and light. If you are adding hops to any meads, be sure to protect the bottles from light by storing them in a dark place, preferably in a box. Sunlight and florescent lights are particularly harmful.