In the chapter on "Treebeard" in The Two Towers, we find that Treebeard, upon first encountering the Hobbits, has no recollection of their kind in the "old lists" the Elves had made regarding the creatures of Middle-Earth.
The reasonable assumption behind this question is that Treebeard was not forgetful, and in fact there was no listing of Hobbits in the "old lists" he learned when young (which, considering he is [one of] the oldest living thing[s] in Middle-earth, is a long time ago). Merry is in fact not surprised, for he notes:
We always seem to have got left out of the old lists, and the old stories.
So Pippin even offers inserting a new line:
Half-grown hobbits, the hole-dwellers.
Put us in amongst the four, next to Man (the Big People) and you've got it
Of course, putting them in that location means the list wording would have to change to "five," not "four" free peoples. But the fact that Treebeard was not forgetful is confirmed when during the Entmoot, no others are able to place them, and in fact they vote to add a new line:
They [the Ents] have agreed that you are not Orcs, and that a new line shall be put in the old lists.
So my question is, why did the Elves leave out the Hobbits when giving the list to the Ents? Are Hobbits a younger creation than all other creatures included on that list? Was there some reason the Elves did not include them at all, but especially under the free people groups? Does Tolkien ever address this further?