This is an interesting article about overthinking; it's not exactly about anxiety, but it does skirt around the topic. Overthinking is quite common among us anxious folk - we love to overanalyze things that have already happened or, even more fun, things that have yet to happen and may never happen.
According to this article, and the several noteworthy academics that it cites, overthinking tends to occur more and more as we age because we have more knowledge and experiences to weigh our current situation against. One interesting example that the article describes is a 6-year old girl who is asked to draw a picture of God and proceeds to create a very imaginative drawing that she is quite confident looks exactly like God. If an adult were asked to do the same thing, 1) the drawing would be far less imaginative/creative and 2) we would be far less sure that our portrayal was accurate. As adults, all of our religious and personal experiences and our self-doubts about our artistic talent flood our brains, and the simple task of thinking about what God looks like and putting it on paper becomes a much more complex task. In short, we overthink the heck out of it.
Most everyone overthinks more as they grow older because our brains have a much greater catalog of experiences to analyze for similar situations. If your anxiety is getting away from you a bit and your find yourself overthinking a situation or decision, try to disengage what is called your explicit memory. Explicit memory are all the previous experiences and rational thoughts that we engage in trying to make a decision. In contrast, implicit memory draws from our gut reactions to a situation and is somewhat more immune to overthinking. We've all used the trick for making a tough choice where you tell yourself, "Okay, I'm going with Option A," then all of a sudden you realize "Oh no, I should have gone with Option B after all!" and voila, there's the correct choice. That sudden realization is you engaging your implicit memory. So in short, if you are starting to overthink, with or without anxiety, remember to trust your gut.