I agree that the aged character is oxidation. I recently tapped a Dopplebock that was brewed using low oxygen methods it has sat for a good 6-8 months in the spunded keg, upon looking at the DO reading it was 0.00, so no oxidation had taken place. However knowing this I made my own oxidation flavors with malts. I blended a range of cara's( I think it was like 5 separate caras) using the weyermann flavor wheel( great resource) to achieve the aged flavors I wanted. It turned out freekishly well. All of the beautiful dark fruit, raisins, etc with none of the soy sauce, kibble, and sherry. So thats an option as well.
Interesting that you mention a kegged beer. I brewed a very big barleywine a few years back, and left it in a purged keg for an extended secondary. I doubt that I started with zero DO, but I'm pretty sure that long term oxygen exposure was significantly lower than in a bottle with a crown cap. After about a year and a half in the keg, the barleywine still tasted a lot younger than I would have expected. I bottled at that point, and it wasn't until about another year or so in bottles that it has finally started to pick up the sherry character that I was looking for.