Complexity with Aging?

I've just tasted my second batch on bottling, a bitter and tasted my first batch after three weeks in the bottle, a stout and am wondering whether beer increases in complexity with aging? By complexity, I mean that certain intended and pleasant notes will develop and become more distinct or whether they generally mellow out and become more of a homogenous flavour? It seems as though with my stout the taste was stronger all around when it was being bottled and now that it has sat for three weeks that it seems a bit week.

I would say yes, it becomes more mellow and homogeneous, and yes, it will become more complex. I find that the mellowing and mingling of flavours actually let's you search and find more in the palate of a beer. When it's a bit young, things are more obvious and powerful, but really, you can't find subtle flavours because of it.