I just had a question about aged guitars, how do they actually age, how does the finish slowly come off, and such things?
I know that people let their guitar get reliced or do it themselves, but I just wanted to know, what exactly causes these aged looks "naturaly".
Actually playing the guitar? Dings? Scratches? White paints tend to yellow a bit with time (oxidation). I'm guessing clear coats become a tad less clear (cloudy). My acoustic with a gloss finish is starting to show a lot of wear on the top in places like where my palm and forearm rests.
'Aging' on guitars is caused by things like physical dings/dents/trauma, acids in sweat causing discolouration, UV damage causing clearcoats to yellow, changes in humidity and temperature, cigarette smoke, harsh solvents that can react adversely to finishes, abrasion to finishes due to physical handling, the list goes on.

To really understand what happens to finishes over time, you need to understand the specific chemistry of the type of finish in question and how the different things that cause it to 'age' are chemically altering it. If that is the scope of your question, then you need to ask an organic chemist.

You need to be specific as to what process of 'aging' you're asking about. Because an explanation of the processes that can take place that cause a guitar to cosmetically look 'aged' are worthy of quite the novel.
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Quote by Axelfox
Nitrocellulose-painted guitars probably show the greatest amount of "aging" among vintage guitars. Part of the problem is that nitrocellulose, which is made by combining nitric acid and cellulose (usually cotton) in the presence of sulfuric acid, continues to break down for most of its lifetime.This begins to happen within the first few years, and is one of the reasons it was dumped by carmakers in the 1950's as a paint for cars. It discolors, reacts with other materials and chemicals, decomposes in the presence of UV, cracks, checks, chips, chalks and wears fairly easily. Nitrocellulose is one of the first plastics, and a wide range of objects have been made from it, including early pool balls, knobs, casings. On guitars, you'll often find it in inlays ("pearloid"), tuner buttons, pickguards and more. The fact that it sort of rots is one of the ways that experts verify the age of old guitars (including the famous '59 bursts). Tuner buttons turn brownish, and distort, inlays shrink and turn colors, and pickguards will sometimes literally turn to powder and outgas nitric acid, which attacks paint, metal parts, frets, strings and even pickup covers on guitars left in the case where the outgassing can accumulate.

Other signs of aging include corrosion of metal bits due to sweat, humidity and temperature, loss of plating on metal parts, etc.. In fact, weak electrical currents that develop between strings and frets have been known to turn frets greenish.

Wood expands and contracts and develops cracks if not properly humidified and the sap on the inside of some woods will crystalize, changing the wood's charactteristics (and not always for the better).

And so on. I have a '39 Epiphone Emperor that looks pristine (it has a french polish finish), a mid '70's L5-S and an L6-S that look brand new, so it's not *just* age that makes a guitar look old.
Although this came up in an electric forum, I'll note that the timber aging noted by dspellman  is well-documented to have a marked effect on tone in acoustic guitars, IIRC (which I might not), something to do with polymerisation of hemicelluloses; finish aging might also contribute to tonal changes in acoustics. They also suffer another age-related effect - their geometry deteriorates due to string tension.