If you go back to the page your question is on and look fourteen questions further down, you'll find the same query. Clicking on that, you will see three of us attempted an answer. The problem is that we all came up with different ones! Also, if you do a Google search, you'll find several other cathedrals are suggested as the oldest, including Wells, Ripon and St David's. It seems, basically, to depend on what exactly is meant by 'oldest'. Do we count ruins of older churches still to be seen below the present building? Take your pick.