How old is old earth creationism?

Nobody in ancient times could have imagined that the earth was billions of years old, so you won't see any explicit attempts to reconcile the Genesis creation stories with an old earth. However, the early Christians did see discrepancies that made them question how literally the creation stories should be understood.

Second century Christian apologist Justin Martyr wrote in his Dialogue with Trypho, chapter LXXXI:

For as Adam was told that in the day he ate of the tree he would die, we know that he did not complete a thousand years. We have perceived, moreover, that the expression, 'The day of the Lord is as a thousand years,' is connected with this subject.

You can see already the seed of the notion that the "days" of Genesis 1 do not refer to 24-hour periods.

Third century scholar Origen of Alexandria went further in De Principiis IV.16 [emphasis mine]:

For who that has understanding will suppose that the first, and second, and third day, and the evening and the morning, existed without a sun, and moon, and stars? and that the first day was, as it were, also without a sky? And who is so foolish as to suppose that God, after the manner of a husbandman, planted a paradise in Eden, towards the east, and placed in it a tree of life, visible and palpable, so that one tasting of the fruit by the bodily teeth obtained life? and again, that one was a partaker of good and evil by masticating what was taken from the tree? And if God is said to walk in the paradise in the evening, and Adam to hide himself under a tree, I do not suppose that any one doubts that these things figuratively indicate certain mysteries, the history having taken place in appearance, and not literally.

Even ancient scientists understood that "evening" and "morning" were meaningless without reference to the sun, moon, and stars; Origen interpreted Genesis in light of the scientific knowledge of his day, paving the way for later generations to reconcile faith and science.

The influential 5th century scholar Augustine of Hippo, in The City of God, 11.6, refused to speculate about what "day" meant in Genesis 1 [emphasis mine]:

For that which is made in time is made both after and before some time,— after that which is past, before that which is future. But none could then be past, for there was no creature by whose movements its duration could be measured. But simultaneously with time the world was made, if in the world's creation change and motion were created, as seems evident from the order of the first six or seven days. For in these days the morning and evening are counted, until, on the sixth day, all things which God then made were finished, and on the seventh the rest of God was mysteriously and sublimely signalized. What kind of days these were it is extremely difficult, or perhaps impossible for us to conceive, and how much more to say!

In The Literal Meaning of Genesis (PDF), 1.19, Augustine gave advice for interpreting the Scriptures' statements about the physical world [emphasis mine]:

Usually, even a non-Christian knows something about the earth, the heavens, and the other elements of this world, about the motion and orbit of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and the seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he holds to as being certain from reason and experience. Now, it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking non-sense on these topics; and we should take all means to prevent such an embarrassing situation, in which people show up vast ignorance in a Christian and laugh it to scorn.

Lest he be misunderstood, Augustine continued [emphasis mine]:

The shame is not so much that an ignorant individual is derided, but that people outside the household of the faith think our sacred writers held such opinions, and, to the great loss of those for whose salvation we toil, the writers of our Scripture are criticized and rejected as unlearned men. If they find a Christian mistaken in a field which they themselves know well and hear him maintaining his foolish opinions about our books, how are they going to believe those books in matters concerning the resurrection of the dead, the hope of eternal life, and the kingdom of heaven, when they think their pages are full of falsehoods on facts which they themselves have learnt from experience and the light of reason?

Later in the same book, Augustine laid out his hermeneutic for reading Genesis (The Literal Meaning of Genesis, 2.9, no online link available, but quoted in Wikipedia, Allegorical interpretations of Genesis [emphasis mine]:

With the scriptures it is a matter of treating about the faith. For that reason, as I have noted repeatedly, if anyone, not understanding the mode of divine eloquence, should find something about these matters [about the physical universe] in our books, or hear of the same from those books, of such a kind that it seems to be at variance with the perceptions of his own rational faculties, let him believe that these other things are in no way necessary to the admonitions or accounts or predictions of the scriptures. In short, it must be said that our authors knew the truth about the nature of the skies, but it was not the intention of the Spirit of God, who spoke through them, to teach men anything that would not be of use to them for their salvation.

So although old earth creationism is a modern view, it draws (as does theistic evolution) on the ancient tradition of understanding Genesis in light of our current knowledge about the universe.