Where are the oldest parts of the oceanic crust found?
The oldest parts of the oceanic crust are found farest from the mid ocean ridges at subduction zones and continental shelves.
New ocean crust is formed at the mid ocean ridges. The new crust is then pushed away from the ridge as newer crust comes to the surface. The ocean crust then spreads out enlarging the ocean.
The farer away from the ridge the ocean crust is the older the crust is. The oldest crust is at edges of the ocean. One place where the crust is the oldest is at edge of a subduction zone. It is here that the oldest ocean crust is pushed under a continental crust and destroyed.
In the image below, the oldest oceanic crust is pink/purple. As you can see, the newest crust (red) is adjacent to where there is seafloor spreading.
The continental shelves like the Eastern coast of the United States is another edge. The continents of Europe and North America are thought to have been connected at one time. A rift valley split the two continental crust apart forming an ocean between them. The continental shelf is where the new ocean started. This is where the oldest ocean crust exists.
Note no where on earth can the are the thick layers of old ocean sediments found in the geological column presently be observed.
The theory of plate tectonics does not seem to allow for the formation of thick deep ocean sediments.