As others have said, the aging process is relatively straight forward. You basically do the things to a document that would actually be done to a document over its lifetime. The difference is you do it all at once, instead of over a period of decades. (I say "straight forward" but there's an art to it, and making it look good takes some skill).

Incidentally, in the film industry we call it "breakdown". Oh, and just for some trivia, the difference between the "regular" Thorin map (which is USD$29.99) and the replica one is considerably more than just breakdown. They're noticeably different in terms of quality in every aspect, not to mention the regular art print map has all of the copyright information on it! Of course, the other thing to bear in mind is that film art and costume department finishers (the people who break down props, sets and costumes) are much better at breaking things down than either you or I, so a big part of that price is paying for the expert work that actually makes the map look like it really is hundreds of years old and not just a picture someone screwed up and spilled some coffee on!

FYI, I've seen Thorin's map and it's a beautiful piece of work. I was highly tempted to buy one. Definitely worth $200 for one mountain and a dragon.