It may be wear to the internal parts but it could also be the result of using filters IF you have them installed to protect the front element.IF you have filters on the lenses REMOVE them & test for better results. If you don't have filters the best advice I have to isolate whether it's a lens problem will be to mount your camera & each lens on a tripod & try using AF for a few test shots (move the lens out of focus after each shot so the AF will have to do it's thing for each shot) and then try a few shots using manual focus, preferable tethered to a computer in live view to see if you get a better result.
If you get a better result in MF mode then you'll need to see whether the problem has more to do with the lens or the body, which will require having at least another lens that nails the focus in AF mode consistently using the same test. If the body has the issue the lens won't be consistent (but may be close to in focus if it has a smaller max aperture). If you have a friend with a Canon body that seems to be working properly re run the tests using it too because a lens problem will go to whatever body the lens gets mounted on.
"A skill is developed through constant practice with a passion to improve, not bought."
