Eldest?

[Raynor]

According to the Silmarillion, the ents awoke about the same time with the elves:

Originally Posted by Of Aule and Yavanna

When the Children awake, then the thought of Yavanna will awake also, and it will summon spirits from afar, and they will go among the kelvar and the olvar, and some will dwell therein, and be held in reverence, and their just anger shall be feared

while Tom "was here before the river and the trees", as Texadan quoted from The Old Forest.