Originally Posted by Revvy
The IBUs won't change with aging- how bitter the beer is.
The IBU's may not change but the perceived sweetness does. Depending on the time frame you may want to over hop the beer. This all trial and error though, like revvy said there's no good calculation for it. It might not matter to you or you might not even taste it.
As far as aroma hoping, I would suggest if possible, bulk aging the beer and dry hopping it 3-4 days prior to bottling. Fortunately/unfortunately all beers flavor changes with time and it needs to be accounted for, and aromas are the first thing to go.
If your worried about tannins, use a 6 row malt for the base and make sure lots or air is incorporated at mash in, as well as a proper pH (around 5.3). This will add a slight red color, but the tannins will precipitate out with proteins in the 6-row, if your using dark malts who cares about color.
(As far as the IBU comment I think it does fall slightly over long periods (6+ months) but I can't find the info to back that up)