Yes certainly if you mean predates them in Middle Earth. It’s more tricky if you mean whether he was in Middle Earth before the Istari (wizards like Gandalf, who were also Maia) existed. I don’t think we can know the answer to that. Unless “eldest” means he predates the entire cosmology.

But if it just means eldest in middle Earth well we also know he is older than the rivers, trees, rain, and acorns. I don’t think we can say much more than that, as he doesn’t say that he is younger than anything.

In response to Sid Kemp's answer to Does Tom Bombadil predate the Istari?, I don’t think Tom Bombadil can be a Maia because that doesn’t explain why the ring had no power over him when it does over other Maia.

Part of the motivation for him, I think is Tolkein’s love of the “flawed narrator” perspective in his books. He’s an element that we just don’t know, he might even be as much a mystery to Tolkien himself, and within the context of the books and the fictional narrators / authors of the books - the narrator doesn’t know either, and he is also a mystery to other characters in the book who also don’t know. If so, we’ll never know, and I rather like it like that myself :).

The rest though, of Sid Kemp's answer to Does Tom Bombadil predate the Istari? I agree with. It’s just about him being Maia - how can he be uninfluenced by the ring if that was true? Gandalf for instance said the temptation would be far too great to carry it himself.

But for Tom Bombadil it’s just a bauble. He would be a most unsafe guardian as he’d soon forget about it and soon lose it. And it has no power over him - when he wears it, he doesn’t vanish, instead, he makes the ring vanish instead momentarily, a fun reversal of ideas.