Eldest?

Originally Posted by FOTR

"Tell me, who are you, alone, yourself and nameless? But you are young and I am old. Eldest, that's what I am. Mark my words, Tom remembers the first raindrop and the first acorn. He made paths before the Big People, and saw the little People arriving. He was here before the Kings and the graves and the Barrow-wights. When the Elves passed westward, Tom was here already, before the seas were bent. He knew the dark under the stars when it was fearless- before the Dark Lord came from the Outside.

If Tom Bombadil were referring to Melkor, than hands-down Bombadil is older than Treebeard. And even with Sauron, since Sauron arrived at generally the same time as Melkor.