Its not clear. There are various opinions about when Abraham began to recognize G-d. Two of them are age 3 and age 48. So any time after that? The passage isn't understanding the Hebrew word "old" to mean "old age" but to mean as an "elder" or "sage".
It would have had to been at least before (depending on how one interpreted the Midrash based on the others) but possibly after as well. Unless this opinion doesn't argue with the other in which case its just after.
This site has over 76,000 Judaic books. Do you really think I could answer that question? Among the sources that I looked, this was the only one that mentioned this Midrash. I do see that the book
Raziel also considers itself to have been passed down through the generations from Adam to Abraham (ostensibly by way of Shem) and the
Or HaChaim also takes mentions that Abraham learned from Shem although he doesn't quote the Z"C or the Raziel.
It means he figured it out himself. According to a
theological book, before the Torah was given at Mt. Sinai, the way to acquire the Torah was to first purify oneself (from bad traits and sin and attaching oneself to G-d) and then based on the degree of purification one would gain prophetic inspiration that caused one to inherently know the Torah (now its the opposite) based on the principle "the Holy One Blessed be He and the Torah are one".
How would one know the correct method to enter the prophetic state without having studied it? I wrote about that a while back.