Is it permissible for a congregation to call a minor – a boy under the age of Bar Misva – to the Torah for an Aliya?
The Gemara in Masechet Megila cites a Berayta which states explicitly that a child may be called for one of the seven Aliyot on Shabbat ("Ha’kol Olin Le’minyan Shib’a Afilu Katan Afilu Isha"). Hacham Ovadia Yosef (listen to audio recording for precise citation) ruled that this Halacha applies not only to the seven Aliyot on Shabbat, but also to the three Aliyot on a weekday. As long as the child has reached the stage of "Onat Ha’pe’utot," the age at which he understands and can participate in the prayer service, and especially if he is near the age of Bar Misva, he can be called to the Torah when the need arises. For example, if a family comes to the prayer service on a weekday morning to celebrate a boy’s Bar Misva several days or weeks before the thirteenth birthday, such as on the day he begins wearing Tefillin, he may be given an Aliya.
Hacham Ovadia extends this Halacha to a case where the only Kohen in the synagogue is a minor. The Magen Abraham (Rav Abraham Gombiner, Poland, 1637-1683) ruled (282:6) that the Misva of "Ve’kidashto," which requires according honor to a Kohen, does not apply to a Kohen younger than the age of Bar Misva. Hence, if the only Kohen in the synagogue is a minor, he is not called to the Torah for the first Aliya. Instead, an ordinary Yisrael is called to the Torah. Hacham Ovadia, however, disagrees. He rules that if there no adult Kohen in the synagogue, and there is also no Torah scholar, then the congregation should call the child Kohen, assuming he is at an age where he understands and can participate in the prayer service, such as around the age of ten or above. Hacham Ovadia notes that this is ruling of several Halachic authorities, including the Erech Ha’shulhan (282:6), the Be’er Ha’mayim (10), and the Mishpeteh Uziel.
Summary: If a congregation wishes to give an Aliya to a boy that has yet to reach the age of Bar Misva, such as to give him honor shortly before his Bar Misva, he may be given an Aliya. If the only Kohen in the synagogue is a minor, he can be called to the Torah for the first Aliya, assuming he is old enough to be familiar with the procedure of receiving an Aliya.