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.