Treebeard is 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 latter:

>Then when the Darkness came in the North, the Entwives crossed the Great River... 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.

Information on 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.