Terumos, Chapter 1, Mishnah 3. In the first Mishnah we listed five categories of people who may not separate Terumah, and even if they did so their Terumah is not valid, and one of them is the minor. According to the Tanna of that Mishnah, a person is not considered an adult and does not meet the Torah's requirement of being an "ish," a man, unless two conditions are met:
Chronological age: for boys, thirteen years; for girls, twelve years.
The onset of physical maturity: generally measured by the appearance of two hairs.
That is the position of the Tanna Kamma. In our Mishnah we will bring two opinions that disagree with him, holding that even before a minor has reached these two milestones - the age and the appearance of the signs of maturity - he can still separate Terumah validly.
The opinion of Rabbi Yehudah:
Rabbi Yehudah says: "katan shelo heivi shtei se'aros" - a minor in whom the physical signs of maturity have not yet appeared, and nevertheless his Terumah is valid Terumah. If he separated, the separation takes effect, the food becomes permitted to eat, and the portion that was separated becomes forbidden to everyone and restricted to consumption by kohanim alone.
Most learners understand Rabbi Yehudah's words - and this appears to be the primary explanation - as referring to a child who has already reached the chronological age: the boy is thirteen and the girl is twelve, but they have not yet reached the onset of physical maturity. No pubic hairs have appeared, nor the other signs found in girls, and even so Rabbi Yehudah holds that this is enough for the separation to be valid. His reasoning is that Rabbi Yehudah does not expound the phrase "kol ish" in the verse dealing with the contributions to the Mishkan as excluding minors.
The opinion of Rabbi Yosei:
Rabbi Yosei is even more lenient: "im ad shelo ba le'onas hanedarim - ein terumaso terumah, umisheba le'onas hanedarim - terumaso terumah" - as long as the minor has not reached the age called 'onas hanedarim,' the stage at which a person can impose a vow, he also cannot separate Terumah. But once he has reached onas hanedarim, his Terumah is valid Terumah.
According to Rabbi Yosei, there is an age one year before the bar or bas mitzvah, meaning a boy after his twelfth birthday and a girl after her eleventh birthday, at which we assume that they know what they are talking about and have the power to impose a vow. They can say that a particular item is forbidden to them as though it were a korban - to look at a pen and say that this pen is forbidden to me like a korban - and the vow takes effect, and takes effect on a Torah level, such that the child who declared the vow may not derive benefit from that pen.
From here Rabbi Yosei reasons in the same way: a person one year before his bar mitzvah can impose a vow and give an object the status of sanctity as though it were a korban; and similarly he has the power to impose the lighter sanctity of hekdesh - to create in an item of food the status of Terumah that is given to a kohen. This ability is his already in the year before he becomes a bar mitzvah or bas mitzvah. Rabbi Yosei too does not expound "kol ish" as coming to exclude minors, and in his view the same mechanism that works for imposing vows works for imposing Terumah as well.
The halachah:
The Bartenura rules like Rabbi Yosei, and in this he follows the Rambam, and the Mechaber likewise follows the Rambam - and therefore for Sephardim this is certainly the law. The Vilna Gaon disagrees and learns that the halachah follows the Tanna Kamma, and the 'Derech Emunah' of Rabbi Chaim Kanievsky brings the Vilna Gaon along with many Acharonim who lean his way. It therefore appears that Ashkenazim should be stringent and assume that without a full bar mitzvah or bas mitzvah one should not separate Terumah.
In summary: In this Mishnah the Tannaim disagree about the stage at which a minor may separate Terumah: the first Tanna requires both the age and the physical signs of maturity; Rabbi Yehudah validates one who has reached the age even though he has not yet grown two hairs; and Rabbi Yose goes even earlier and validates one who has reached the age of vows - a year before bar or bas mitzvah - since the power to make a vow take effect is also the power to make Terumah take effect. In practice the halachic authorities are divided: the Rambam, the Bartenura and the Mechaber rule like Rabbi Yose, while the Vilna Gaon and many Acharonim rule like the first Tanna.