my guess is it has to do with 'complex' chemicals breaking down...chemicals that normally lend 'off flavors' to the wine breaking down into flavorless compounds so the true flavors come out.
purely a guess though.
When aging a lot of chemical processes occur in a wine.
For example. We all know invert sugar which is made from heating table sugar. The table sugar will split in glucose and fructose.
Now when there is residual sugar in a wine this process will occur also. Only it takes a lot longer as there is no heat to speed it up. So that would alter taste a tiny bit in the long run.
Next bindings take place.
Alcohol and acid will bind and will form esters.
Again a process that takes a long time but will certainly impart
flavors in a wine that were not present before.
Many more bindings will occur which impart flavor changes.
Tannin will drop out. Therefore the wine will mellow out.
Maybe some tartaric acid will drop out which brings down acidity
Small doses of oxygen will get into the wine (micro-oxygenation) through the cork. This will in due time oxidise the wine. But in the mean time (many years with a good cork) a very small bit of oxygen will get in. It will bind with components in the wine and alters taste and flavor.
A few hundred other processes are going on while the wine ages.
That is why a wine will change so much after years of aging.
I make dandelion wine.
After a few weeks when fermentation has finished it tastes awfull. But man how that changes in one years time. That is why I still make it.......
I have a beet wine that taste like eating mud after fermentation stopped. After a year it got a bit better. It will be drinkable in about 3 years !!!!
Do yourself a favor. Take a bottle of your best wine and just put it away for 2 years then taste again. You will definately be surprised.
That might be a pretty safe assumption for the vast majority of wines made from kits and juice (no skins) but considering most commercial red wines aren't even released until they are two years old, wines made from fresh grapes can vary greatly in how long they take to mature. I had the opportunity this past fall to taste some extremely fine examples of Pinot Noir from Burgundy (the complete DRC line) from the 1996 vintage. The Grands Echezeaux and Echezeaux were drinking beautifully and were likely at their peak, while the Richebourg and La Tache were still many years from hitting prime time. Same vintage, same winery, same grape variety, but very different wines from each vineyard, and some requiring much more time than others.