A few quotes from the Rabbi on 'Traditions of the Fathers'
in which he replies that even a boy of thirteen and a day is worthy of praying at the teiva
In places where there is no shaliach tzibbur worthy of leading the congregation in prayer and there are youths of thirteen or fourteen years of age, is it permitted for them to lead the congregation to fulfill the public's obligation in kaddish and kedusha, so that the congregation do not avoid [the obligation of] public worship? Instruct us, and be recompensed by heaven.
Know that a youth of thirteen and a day, even if the last day is a Sabbath and he has not yet worn phylacteries, is considered a grown man, and may be called to the Torah Reading, read the Torah and pray as shaliach tzibbur. This will not be asked of him before he wears phylacteries, has come of age, and has two hairs and is considered an adult for all matters.
Yad Eliyahu, Part A, Question 27, p. 26a-b, Shemesh David Torah Library, Jerusalem, 1992