[Wordsmith]

Not to jump ahead, but I believe Gandalf refers twice to Treebeard as the oldest living creature in Middle Earth. A slip by Tolkien? Or here's a wild possibility. Gandalf, at Rivendell, mentions having "completely forgotten" about Bombadil. Is it possible that there is something about Bombadil that keeps him from remaining in mind, even for Gandalf? Is this why Gandalf, mistakenly, calls Treebeard the eldest?

[Overtaxed]

A slip by Tolkien?

Now you're treading on dangerous ground! :)

You're explanation sounds as good as any. Apparently Tolkien meant for Bombadil to be a mysterious character but I hate it when they're this mysterious. To me, all these questions lead back to "Who or What is Tom Bombadil?"