I looked it up and it's cìrdan

Sauron is older than Galadriel, but I assume that you are not including him since he is also an Aniur. People might be confused about not including the Istari so if it's multiple choice maybe help them out by not including the wizards at all. Good luck with your quiz!

Don’t forget about the ancient dragon Smaug

“Treebeard...the oldest living thing that still walks beneath the Sun upon this Middle-earth.”

I feel like Cirdan would be the eldest. He was born before Galadriel and was one of the first Sindarin Lords (I think he was a Sindarin Lord? I may be confused.)

Treebeard.

Huh, truly, Treebeard is older than Cirdan.

Círdan originated in Cuiviénen, ~YT1050; the ents were given life by Eru after a request by Yavanna to protect the trees from other creatures. Manwë tells her the ents would awaken when the Elves awoke. This statement would weigh toward Cirdán being slightly older.

Hm, OK. Nice.

In “Of Aulë and Yavanna” this is said: “Behold! When the Children awake, the the thoughts of Yavanna will awake also, and it will summon spirits from afar, and they will be among the kelvar (animals) and olvar (plants), and some will dwell therein, and beheld in reverence, and their just anger shall be feared. For a time while the firstborn are in power, and while the secondborn are young.”

So this suggests at least after the elves, the ents were awakened. Círdan was a Teleri elf from Cuiviénen who did not go to Valinor. This statement also suggests the ents were awakened after Men, so maybe early FA.

Either way, I’m going to lean Círdan, because the firstborn are first for a reason, that being said, Círdan has barely any representation in the films, so maybe Treebeard.