As promised, here is my old photo tutorial, outlining the various steps and tricks that I use when aging photos. Enjoy! :)
The photo that I'm using here is one of my Civil War portraits of a Union medic.
The first step in my aging process is to convert the image to B&W. I always use channel mixer for this step, but any B&W technique will work. Occasionally, I will do an old photo in color, but not often.
Now is also when you should clone out any modern objects that you don't want in the photo, like power lines, cars, etc. In this case there were none; however, I did replace the BG because during the Civil War a portrait like this would have been taken in a studio rather than in the field.