Tolkien was human. Humans make mistakes and oversights.
Indeed. He could also have kept Aragorn as a hobbit named Trotter instead of a human and now we would be debating a number of different inconsistencies.
Who's the eldest being in Middle Earth, Tom Bombadil or Treebeard?
It would seem pretty obvious that Treebeard cannot be older than Tom Bombadil (who claims to remember "the first raindrop and the first acorn"). I would hardly consider this a "most noticeable inconsistency"