Way out of my league, but there are several things going on with age. Whether the sugar degrades or not is actually an open question as far as I know, but our perception of it changes because the wine changes. When we smell strawberries, we expect sweet, so we taste sweet. When we smell green peppers, we don't expect sweet, but even with the same amount of sugar present, they won't taste as sweet. So a Grenache with less sugar than a Merlot might actually taste sweeter to us.
Same with sweet wine. As the wine ages, it oxidizes a bit, the chemistry changes, and the volatile compounds change and even with the same sugar level, it tastes less sweet. The weight of the wine is partly due to dissolved solids and partly due to other compounds. Oxidation of glucose produces glycerol, which has an effect on the mouth feel, but there's also oxidation of tartaric acid and other things that completely change the aromatics of the wine and also the texture of it. The oldest sweet wines I've had that were not fortified have seemed lighter in weight than their younger counterparts. Some are still pretty good though!
I'm not a fan of that thick syrupy quality in young Sauternes actually.