In the Sillmarillion, there were a huge number of creatures who participated in "The Music of the Ainur," which foreshadowed the creation and history of Earth.
Many of these entered Arda and dwelt there when it was created. The 15 most powerful were the Valar, the single most powerful of whom was Melkor, the original Dark Lord, of whom Sauron was only a servant.
I would assume Tom B. was one of these Ainur, who were called the Maiar. They were mostly the followers and people of the Valar, but Tom was probably a free-lancer. If so, he was older than Middle Earth or Arda itself. Sauron, Saruman and Gandalf were other Maiar who were presumably older than Arda. Although some Maiar reproduced, so some of them might not be older than the Earth.
Treebeard was born(?) on Middle Earth and at the end of the Third Age might be the oldest "native" still alive.