The Tziyun Yerushalayim to the Yerushalmi Sotah 21 points out that the Rishonim differ as to the source for the shiur gadlus for a boy or girl being age 12 or 13. Rashi (Nazir 29) writes that Levi is called an "ish", a mature adult, when he was 13 years old, as we see in the pasuk describing the battle of Ya'akov's children against Shechem: "Vayikchu shnei Bnei Ya'akov, Shimon v'Levi, ish charbo..." (Braishis 34). The Tshuvos haRosh, however, writes that shiur of gadlus is a halacha l'Moshe m'Sinai without a textual basis.
The T"Y suggests a possible nafka minah between these views could be whether the shiur gadlus is constant or variable. According to Rashi, 12/13 is a constant. According to the Rosh, perhaps in earlier generations in Sefer Braishis where the life cycle was different and the mind matured earlier, the shiur gadlus was younger as well.
I am not sure I fully understand why the T"Y assumes that if the shiur gadlus is a hllm"s, it follows that it is a relative number. I would suggest a different possible nfk"m. Achronim debate whether this shiur of 12/13 applies to bnei noach or only to Bnei Yisrael. Perhaps the answer hinges on this debate between Rashi and the Rosh. If the shiur gadlus is rooted in halacha l'Moshe m'Sinai like the Rosh suggests, hllm"s is binding only on Bnei Yisrael. If the shiur gadlus is derived from the age of Levi like Rashi suggests, then if one assumes the children of Ya'akov did not fully emerge from having the status of bnei noach pre-mattan Torah (as discussed by the Parashas Derachim and others) there is room to suggest that this shiur is universal.