I think an aging is going to be a non-linear (not in time, but in brightness application) fade to an off white and de-contrasting add addition of grains (not white noise, but bigger grains/dust). What's hard about this is there's some threshold of believability. Real photos age surprisingly well even when very sun bleached, staying pretty visible right till the end. You want slow degradation that does actually remove some of the photo each time, from what I can tell. You might be able to get away with a literal dust layer on top for that though.