We're never given any hints about the age of Wellinghall. We know that Treebeard was in Beleriand during at least some of the First Age, and he must have crossed the Blue Mountains before the end of that Age. We can probably therefore place the founding of Wellinghall sometime in the Second Age (which would make it several thousand years old by the end of the Third) but the lack of hard information makes it impossible to be more precise.

'Wellinghall' was the not the original name of this place, but a partial translation of the original Entish name; that full Entish name is not recorded.

We don't have a map showing exactly where Wellinghall lay within Fangorn, so the location shown here is based on textual references. Specifically, we're told that Wellinghall was about sixty miles from Treebeard's Hill, and that hill was relatively close to the point where the river Entwash flowed out of the forest. We're also told that Wellinghall stood in the foothills beneath the southern Misty Mountains. So, based on all these clues, the Ent-house of Treebeard must have been somewhere close to the point marked on this map.