Terumot, Chapter One, Mishnah Three
This mishnah brings in a debate with regard to terumah separated by a minor.
1) A minor who has not yet produced two [pubic] hairs: Rabbi Judah says: his terumah is terumah.
2) Rabbi Yose says: if he has not arrived at the age when his vows are valid, his terumah is not terumah, but if he has arrived at an age where his vows are valid, his terumah is terumah.
Section one: According to mishnah one above, the terumah separated by a minor is not terumah. Rabbi Judah disagrees. One sign of a child having reached majority age is the appearance of pubic hair. The rabbis usually say that the appearance of two hairs is a sign of puberty. Rabbi Judah holds that with regard to terumah, even if two hairs have not yet appeared, the terumah separated by a minor still counts as terumah.
Section two: According to Rabbi Yose, the ability of a minor to separate terumah depends upon his ability to take a vow. If the child is old enough such that his vows are valid, then his terumah is terumah. For a boy this is at 12 and for a girl this is at 11. At this age the child is assumed to be able to grasp the meaning of vows, and so too, the child is assumed to be able to understand the consequences of declaring something to be terumah.