"It is my understanding that as wood ages the resins polymerize, much like epoxy but over decades or centuries, not hours. "
That's a common belief, but I've never seen any data. I have cut into spruce that was more than 150 years old and found pockets with runny pitch, so I'm not sure about that. There is, however, a documented change that alters the ratio of crystalline to amorphous material in the wood, and reduces the weight: hemicellulose degradation.
There are three main structural components in wood; cellulose , which makes up about 50% of it, and equal amounts of lignin 'glue' and hemicellulose 'filler'. Cellulose is a long-chain polysaccharide; little 'bricks' of sugar that are bound together end-to-end in a specific structure. These chains are organized into bundles that make up the fibers, mostly oriented along the grain. Because of the chemistry cellulose is very stable and hard to break down, and the fibers are strong and don't melt.
The fibers are glued together with lignin, which is, as I understand it, a phenolic resin. It's thermoplastic; softening somewhere around 108F (80C) and discoloring at around 300F (150C). It's also pretty stable chemically.
Hemicellulose is also a polysaccharide, but it does't have a definite structure because it's a 'branched chain' polymer. This leaves the molecules with a lot of loose ends, so to speak, that can react with water. It's the hemicellulose that actually absorbs moisture from the air when the humidity goes up, and causes the wood to swell. I'm told this also increases the damping factor; the rate at which the wood dissipates energy in vibration, and, of course, it gains a bit of weight.
Hemicellulose is not very stable chemically; with moisture cycling it breaks down into the CO2 and H2O that were put together originally to make the sugar 'bricks'. As the hemicellulose breaks down the wood becomes more stable because it can't take up as much moisture. It also loses a bit of weight and stiffness, but the weight goes down faster, so the ratio of Young's modulus to density goes up slightly. Over time this opens up little voids within the wood that reflect light rather than transmitting it, in the way that the air spaces in a snow bank make the transparent ice look white. New spruce tops are translucent, but old ones are opaque.
The loss goes at about 1%/decade, if memory serves, so it's slow. It's still significant, especially in terms of stability. This is the difference between wood that was recently cut and is 'dry', and old wood that has been 'seasoned'. Because hemicellulose loss is driven by moisture and temperature cycling, wood is most quickly seasoned in a place that's dry, but open to the air, so that it sees lots of changes in temperature and humidity. I've got some wood seasoning upstairs in my barn that must be getting better at a fantastic rate!
Hemicellulose degradation is probably also behind the phenomenon of 'shrinkage hysteresis'. Something like a guitar top gets a bit wider as the humidity rises, and shrinks as it gets dry. If you go back and forth from, say, 10% R.H.to 90%, and back, it's been found that the wood gets a little smaller for every cycle; it shrinks a little more when it's dry, and never quite swells back to full size when the humidity goes up. This probably has to do with the loss of hemicelluose and the consequent inability of the wood to take up as much moisture with every cycle.
Since the main 'crystalline' material in wood is the cellulose, and the lignin and hemicellulose are amorphous, as the hemicellulose goes away the ratio changes. It may not be that something 'crystallized', but rather that something that was amorphous went away. This is not to say that the resins can't or don't crystallize, but I haven't seen any evidence of that, and I know that hemicellulose degrades.