Lenses such as your Nikon 18-200 have internal adjustments to allow centering and aligning the various elements and groups in the optical path. Use over time can cause those adjustments to move slightly, but most lenses that are out of alignment enough to be really noticeable are victims of a bump or drop that provides enough shock to cause things inside the lens (hopefully the points of adjustment) to move.

There are also other issues that can affect the sharpness of photos taken with a lens.

• Fungus growing inside the lens can cause a gradual reduction in sharpness as more and more of the optical path is affected by the spread of the fungus.
• If you are using Auto Focus, there may be a problem with the calibration of the AF system in the camera body.
• Due to very minor differences in manufacturing tolerances, the same lens may be sharper on one camera body than another.
• Even a mounting flange on a body or lens that is out of alignment just a few microns (1µ= 1/1000mm) can cause a loss of sharpness, especially at wider angles of view and larger apertures.

Several of these can worsen gradually over time. If the performance of your lens has truly degraded it may be worth a trip to the nearest Nikon Service Center for a cleaning and calibration. Just be aware that the cost for this could approach an appreciable percentage of the lens' value or even exceed it, depending on which version of the 18-200 you own and the overall condition of the lens.

There may also be some selective perception going on here. If I look back at photos I took several years ago and compare them to what I shot yesterday, I often ask myself the same question: "Why were all of the photos I took back then sharper than the ones I took yesterday?" Of course, what is really going on is that I am usually comparing the very best shots from several years ago against the entire group I just shot yesterday. If I go back into my archives and look at all of the photos taken from a particular shoot several years ago, I see all of the rejects that were just as slightly imperfect (or fully missed and totally blurry) as some of the photos from yesterday's shoot.

Especially when using AF, there will be a degree of error from shot to shot. Different camera/lens combination will demonstrate various degrees of standard deviation, even when properly calibrated. Phase detection AF was developed for speed over absolutely perfect focus every shot. Newer camera/lens combinations that allow the lens to communicate back to the camera body exactly how far it moved in response to the camera's instruction are showing significant improvement along these lines, but you must have both a newer camera and a newer lens that both support the feature to get the benefit from either.