It is often said that there are seventy approaches to Torah. What is the earliest known source that says this?
(This related question merely quotes "Bamidbar Rabbah," and I assume he refers to 13:15 16. Is that the earliest source?)
This exact expression is found in the Vilna ed. of Bamidbar Rabbah (13:5) as you note.
It is also found in B'reshit Rabbati (Parashat Vay'chi p. 141) which probably dates back to roughly the same time and place:
שידרוש בתורה בע' פנים מנין יין
An older source is the Midrash Otiot D'Rabbi Akiva (Version A) as found in the Battei Midrashot compilation (vol. II) which states:
מפני שכל גנזי החכמה מסורין בידו וכולן נפתחו למשה בסיני עד שלמדו לו בארבעים ימים כשהיה עומד בהר, התורה בשבעים פנים
This states that all the vaults of knowledge were possessed by [a particular angel] who transmitted them to Moshe who then learned the Torah with 70 faces.
According to Wikipedia Version B dates from at least the first half of the tenth century, and version A is even older.
A similar idea is found in the Talmud (Sanhedrin 34a, cf. Shabat 88b):
מקרא אחד יוצא לכמה טעמים ואין טעם אחד יוצא מכמה מקראות דבי ר' ישמעאל תנא (ירמיהו כג, כט) וכפטיש יפוצץ סלע מה פטיש זה מתחלק לכמה ניצוצות אף מקרא אחד יוצא לכמה טעמים
One verse is stated by God and from it emerge several explanations, but one explanation does not emerge from several verses. Alternatively, the school of Rabbi Yishmael taught that the verse states: “Is not My word like as fire? says the Lord; and like a hammer that breaks the rock in pieces” (Jeremiah 23:29). Just as this hammer breaks a stone into several fragments, so too, one verse is stated by God and from it emerge several explanations.