who's the oldest?

[Lord Gothmog]

Perhaps you are trying to draw some sort of biological distinction between Tom and Treebeard, based on which of the two is closer to the real-world definition of life?

But in that case, you'd have to say that Gandalf is "nonliving" as well.

So far as Gandalf is concerned, he is, with the other Istari, in a completly different class to any other in Middle-earth.

The point of difference being put forward is that between "Living" refering to creatures born of Arda and "Non Living" refering to creatures not born of Arda. The Istari were originaly created before Arda and by this definition would be "Non Living" but as we can see from this Quote from Unfinished Tales about them:

For with the consent of Eru they sent members of their own high order, but clad in bodies as of men, real and not feigned, but subject to the fears and pains and weariness of earth, able to hunger and thirst and be slain; though because of their noble spirits they did not die, and aged only by the cares and labours of many long years.

Therefore the Istari were of both "Living" and "Non Living". However, their "Real and not Feigned" bodies were much younger than Fangorn and Bombadil.