Treebeard is said to be;
'The oldest Ent alive'
This suggests that Treebeard might not have been the first Ent. If he was the first Ent, then he would be at least Three Ages old! This sounds rather unlikely unless Ents are immortal like Elves (see my other question).
So, is Treebeard the first Ent? If not, who is the first Ent?
Gandalf does an excellent job of describing Treebeard in the chapter “The White Rider”
Treebeard is Fangorn, the guardian of the forest; he is the oldest of the Ents, the oldest living thing that still walks beneath the Sun upon this Middle-earth.
The Two Towers, Book III, Chapter 5: The White Rider
We know that ents were created before the first age by the pleading of Yavanna
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.
The Silmarillion, Chapter 2: Of Aulë and Yavanna
While it is unknown when, Treebeard also speaks about the Elves waking the trees and teaching the Ents to talk.
Elves began it, of course, waking trees up and teaching them to speak and learning their tree-talk...
The Two Towers, Book III, Chapter 4: Treebeard
Finally, Legolas and Aragorn describe Fangorn (the forest, although based on Gandalf’s quote also the Ent) as older than Men, while the Elves walked under the light of the Trees alone.
... for Fangorn is old, old even as the Elves would reckon it.'
'Yes, it is old,' said Aragorn, 'as old as the forest by the Barrow-downs, and it is far greater. Elrond says that the two are akin, the last strongholds of the mighty woods of the Elder Days, in which the Firstborn roamed while Men still slept...'
The Two Towers, Book III, Chapter 2: The Riders of Rohan
Based on the first quote, and all the supplementary information, the Ents are likely very old and Treebeard being the oldest has lived to a great age.
Treebeard is at least three ages old, as he relates in the chapter named for him:
"The trouble is that there are so few of us left... Only three remain of the first Ents that walked the woods before the Darkness: only myself Fangorn, and Finglas and Fladrif- to give them their Elvish names: you may call them Leaflock and Skinbark if you like that better."
According to Gandalf, Treebeard is older than Leaflock and Skinbark, so neither is the first Ent despite their extreme age
For Treebeard is Fangorn, and the eldest and chief of the Ents, and when you speak with him you will hear the speech of the oldest of all living things.
The Darkness Treebeard referred to was Morgoth, not Sauron, as clarified a little later. So when Treebeard calls himself one of the first Ents, he claims to go very far back indeed:
Then when the Darkness came in the North, the Entwives crossed the Great River... After the Darkness was overthrown the land of the Entwives blossomed rickly and their fields were full of corn. Many men learned the craft of the Entwives and honoured them greatly; but we were a legend to them, a secret in the heart of the forest. Yet here we still are, while all the gardens of the Entwives are wasted: Men call them the Brown Lands now.
I remember it was long ago-in the time of the war between Sauron and the Men of the Sea-desire came over me to see Fimbrethil again...We crossed the Anduin and came to their land, but found it a desert...
The Brown Lands were ruined in the War of the Last Alliance as said in Letter 144:
I think that in fact the Entwives had disappeared for good, being destroyed with their gardens in the War of the Last Alliance (Second Age 3429 – 3441) when Sauron pursued a scorched earth policy and burned their land against the advance of the Allies down the Anduin...
Information on specific Ents is pretty rare outside these passages, and no other Ents that might be older than Treebeard are ever mentioned. I'm not sure any passages precludes there being an Ent that was older than Treebeard killed before the events of the Two Towers (Fimbrethil could technically be one if Gandalf believes she was dead), but there's no particular reason to assume that either. In any case, Treebeard is one of the first Ents, so none could be that much older if they did exist.
