We can now explain why Rambam (according to Rogatchover’s explanation) chose to align with the opinion that “the age of vows” is specifically when the minor reaches the age of maturity (but has yet manifest physical signs of maturity):
The concept of vows contains a wondrous novelty: A Jew has the power to take something permitted (according to Torah law) and make it forbidden like an offering, to the extent that the forbidden object becomes sacred (similar to the sanctity of an offering).
Moreover, there is an even greater novelty here than in offerings themselves.
Offerings have certain limitations, as only specific species of animals (along with certain vegetation and inanimate objects) are fit to be offered on the Altar. With vows, however, no such limitations exist. Once a person extends the scope of his vow, even to something impure, sanctity takes effect on the vowed object (through his vow).
This results in a paradox: Specifically regarding vows, which require additional strength {to be able to sanctify anything} (greater than any other mitzvah requires), the Torah tells us that even a minor (to whom other mitzvos do not apply) can bring about this sanctity (if he is “a minor nearing adulthood”)!
Rambam therefore maintains (as the Rogatchover elucidated) that it is sufficient to state this novelty with regard to a minor who has reached the age of maturity but (merely) lacks the physical signs of maturity — rather than asserting the more radical novelty: that someone who is {only} twelve years and one day old and “knows how to make a vow properly,” can confer sanctity (to any object)!
References:
- See Likkutei Torah, Matos (p. 82b, 83b, ff.); Tzemach Tzedek’s Sefer HaMitzvos, “Mitzvas Nedarim,” at the beginning and end; and other sources — based on Kesubos 59b, et al. See also Rashi on Niddah 46b, s.v. “terumah.”
- Tzemach Tzedek’s Sefer HaMitzvos, ibid, at the end.
