In this week's Parsha, when Bnai Yisrael are leaving Mitzrayim, they are accounted for by the Torah in the Posuk saying, "כשש מאות אלף רגלי הגברים לבד מטף", meaning that they were about six hundred thousand on foot, including just the men, aside from the little ones (שמות י"ב;ל"ז). Commenting on this Posuk, Rashi suggests that "גברים" men, here, means men above the age of twenty (and not above the age of Bar Mitzvah from ages thirteen to nineteen). Some later commentators explain that Rashi inferred the meaning of גברים here from a Posuk in Parshas BeMidbar, where the Torah tells us that the number of men above the age of twenty who should go to war was approximately six hundred thousand (במדבר א';מ"ה-מ"ו). Since that count took place in the same year as the events of our Parsha, it is logical to assume that both counts must be referring to the same age group, namely, men aged twenty and above. The Gur Aryeh adds that the word גברים itself comes from the word גבורה, courage, and therefore is referring to the brave men of age twenty and up who went to war, and not to all adult males above the age of thirteen.
Rabbeinu Eliyahu Mizrachi, however, remains puzzled by Rashi's explanation of our Posuk, because the Posuk specifically says that this number was of the גברים, and not the "טף," meaning not the children. How could men from the age of thirteen through nineteen be included in the phrase "לבד מטף," not the children? Surely people of that age are אנשים, men, and not טף! The Gur Aryeh attempts to answer this question by stating that the Torah means to say that all the men of twenty years and up are גברים, while the phrase לבד מטף, not the children, simply excludes the main remaining category of males, that is, those below the age of thirteen, even though males between ages thirteen and nineteen also were excluded.
In order to answer this same question, the Netziv cleverly suggests another, very unusual approach. He suggests that the word טף really derives from the word טפל, meaning secondary or additional. What the Torah is really saying here, then, is that there were six hundred thousand גברים, which was the primary group consisting of men above the age of twenty, aside from the additional groups which consisted of all others, including elderly people, women, children and men between the ages of thirteen and nineteen. The Netziv makes a similar comment on the Posuk in Parshas VaYigash (בראשית מ"ז;י"ב) which also uses the word טף, but which likewise seems to refer to more than just children, because the subject there is the people in the family for whom Yosef provided food. Obviously, the adults had to eat too. We therefore see that the word טף can sometimes mean something other than children, and thus Rashi's comment in our Parsha makes good sense.