The Tiferes Yisrael explains that both 'Yeled' and 'Zakein' are relative, and what the Mishnah means is that a person who belongs to any of the younger groups, who undertakes to give the Erech of someone who belongs to an older one, pays the Erech of the older group, and vice versa, which I think is what you are saying.

The Ya'avatz explains that Rashi does not come to exclude somebody from the group of five to twenty (in fact, he explains 'Yeled' incorporates anyone from the age of thirteen whose Neder is valid [not five, which is not]). And Rashi only says from twenty to sixty as a Chidush (I think he means because his Erech is much higher than the thirteen-year old, yet even he pays the lower Erech of the Zakein).

It is also possible to say that, since the Tana is only speaking about someone who is over thirteen, Rashi prefers to establish the case with regard to the older group (of between twenty and sixty), since it incorporates the entire group. In any event, he has no intention of precluding someone between the ages of thirteen and twenty from the Na'ar in the Mishnah.

be'Virchas Kol Tuv

Eliezer Chrysler