[Valpal1]

Somewhere, and I couldn't find it again, someone (Strider, Gandalf or Bilbo) tells Frodo that the firstborn (the elves) call him (Bombadil) Eldest, implying he was there before they awoke, and predates them.

[MozartLover]

Tom tells Frodo that himself in chapter 6: "Eldest, that's what I am......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 Outside."