The gemara (Nidah 46) quotes Rav Huna that if a muflah hasamuch l’ish makes a neder and then violates that neder, he/she would get malkos. If the chiddush haTorah of muflah hasmauch l’ish was just that even a neder of a katan has validity, then Rav Huna’s din wouldn't make sense – a katan is exempt from punishment. It must be that the Torah treats a muflah as a gadol. The same can be seen from the Rambam’s formulation (Nedarim 11:4):
הואיל והגיעו לשני הגדולים, נדריהן קיימין--אף על פי שלא הביאו סימנין, ועדיין לא נעשו גדולים לכל דבר; ודבר זה מדברי תורה--שהמופלא הסמוך לאיש, הקדשו הקדש ונדריו נדרים.
The words “higi'u l'shanei hagedolim" imply that a muflah is considered (viz a viz nedarim) a full fledged gadol.
We discussed a similar issue once before. R’ Chaim (al haRambam) asks how the gemara can have a hava amina that the age at which child can become a ben sorer u’moreh is under 13 – a minor cannot serve as a defendant or plaintiff in beis din! R’ Chaim therefore concludes that the gavra of the ben sorer u’moreh is technically never found guilty of any crime. Beis din simply finds as a matter of fact that he has the status of a ben sorer, and because of that status he is given a punishment for the inevitable future crimes he will commit (ne’herag al shem sofo). R’ Shach in Avi Ezri sees this as a distinction without a difference. How can a person be punished without being guilty of some crime? Rather, explains Rav Shach, even though normally a minor cannot come before beis din or receive punishment, the chiddush of the parsha of ben sorer u’moreh is that viz a viz this particular parsha we treat the individual as a gadol from an earlier age.
My son showed me that the Shitah Mekubetzes in Archin 3a asks why a pasuk is needed to exclude a katan from punishment for the issur of being metamei mikdash – a katan is never liable for punishment? The Shitah gives two answers: 1) since a katan is included in the parsha of tumah, i.e. he can become tamei, I might have thought the issurim and punishments extend to him as well, kah mashma lan; 2) the exclusion is needed for someone who becomes tamei as a katan and then becomes a gadol and enters the Mikdash. Could the two answers of the Shitah hinge on this chakirah? The first answer seems to accept at least in theory that a katan could be liable for punishment, which would imply that viz a viz the parsha of tumah he is treated as a gadol. The second answer perhaps holds that a katan remains a katan, irrespective of whether certain dinim apply to him at an earlier age.
I thought maybe the din of tumas mikdash is different. It’s not an issur gavra of entering the mikdash while tamei, but rather a din in the cheftza shel mikdash not being a place where tumah is brought. It doesn’t matter whether the vehicle for the tumah coming in is through a gadol or a katan. By way of analogy, the S.A. paskens that a katan can light ner Chanukah for the family even though normally a katan cannot be motzi a gadol in mitzvos. There too, the idea may be that the mitvah of ner Chanukah is the cheftza shel menorah being lit (as R’ Chaim al haRambam explains by neiros mikdash), irrespective of who is doing the lighting. Essentially I am trying to have my cake and eat it too – like R’ Chaim explains by ben sorer u’moreh, I am essentially opening the door to giving out a punishment absent any chovas hagavra, simply by virtue of a certain situation being created. Now that I’ve written this out, I’m less convinced that this makes sense, but I’ll throw it out there anyway for your thoughts.