For those who are new to the Valentinians, I’ve discussed their origin myth more in-depth here, but here’s a copy-paste for your convenience:

There are two realms, a realm of the true Divine and a lower realm. Two Aeons (basically primordial ooze deities) exist (Bythus and Ennoea). They produce, through either birth or emanation, a series of other Aeons, for a total of thirty. The last one, Sophia, is too far away to know the Father (Bythus), and she tries to stretch herself toward the Father. She fails because either (1) you just can’t do that or (2) a purifying power called Horos restrains her, and freaks out and brings forth an amorphous substance “such as her female nature enabled her to produce.” So material substance has is beginnings in fear, ignorance, and error.

There’s not really a whole lot on the origin of the name “aeon,” as far as I can tell from a cursory search; the word “aeōn” in Latin is from the Greek “αἰὼν,” which has a range of meanings, according to Middle Liddell (the Intermediate Liddell-Scott Lexicon): life/lifetime (Homer), age/generation (Aeschylus), long space of time/age/forever (Aeschylus), definite space of time/era/epoch (attributed to the New Testament). Middle Liddell also notes a use in the phrase “εἰς τοὺς αἰῶνας,” an idiom used in the NT to mean “forever.” In Latin, Lewis and Short give the meaning of “aeōn” as “eternity” or “age,” and specifically mention that it’s used when describing the Valentinian system against which Irenaeus and the like were writing.

So what does that mean for the gnostics? Pétrement gives us this, citing the gnostic Gospel of Truth and The Tripartite Tractate:

The Valentinian writings explain to us that God drew back, hid himself, that he wished to remain inaccessible to all direct approach, to be separated from the Aeons themselves (that is, the eternal beings derived from him), so that the Aeons would have to return to him after having sought him.

So the aeons are called “eternal” because, well, they’re eternal, and the word and its meanings seem to predate Irenaeus and the Valentinians.