Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon XL

Canon XL.

Since to cleave to God by retiring from the noise and turmoil of life is very beneficial, it behoves us not without examination to admit before the proper time those who choose the monastic life, but to observe respecting them the limit handed down by our fathers, in order that we may then admit a profession of the life according to God as for ever firm, and the result of knowledge and judgment after years of discretion have been reached.  He therefore who is about to submit to the yoke of monastic life should not be less than ten years of age, the examination of the matter depending on the decision of the bishop, whether he considers a longer time more conducive for his entrance and establishment in the monastic life.  For although the great Basil in his holy canons decreed that she who willingly offers to God and embraces virginity, if she has completed her seventeenth year, is to be entered in the order of virgins:  nevertheless, having followed the example respecting widows and deaconesses, analogy and proportion being considered, we have admitted at the said time those who have chosen the monastic life.  For it is written in the divine Apostle that a widow is to be elected in the church at sixty years old:  but the sacred canons have decreed that a deaconess shall be ordained at forty, since they saw that the Church by divine grace had gone forth more powerful and robust and was advancing still further, and they saw the firmness and stability of the faithful in observing the divine commandments.  Wherefore we also, since we most rightly comprehend the matter, appoint the benediction of grace to him who is about to enter the struggle according to God, even as impressing speedily a certain seal upon him, hereupon introducing him to the not-long-to-be-hesitated-over and declined, or rather inciting him even to the choice and determination of good.

Notes.

Ancient Epitome of Canon XL.

A monk must be ten years old.  Even if the Divine Basil thought the one shorn should be over seventeen.  But although the Apostle ordains that a widow to be espoused to the Church must be sixty, yet the Fathers say a Deaconess is to be ordained at forty, the Church in the meanwhile having become stronger; so we place the seal on a monk at an earlier age.

Aristenus.

The eighteenth canon of Basil the Great orders that she who offers herself to the Lord and renounces marriage, ought to be over sixteen or even seventeen years of age:  so that her promise may be firm and that if she violates it she may suffer the due penalties.  For, says he, children’s voices are not to be thought of any value in such matters.  But the present canon admits him who is not less than ten years and desires to be a monk, but entrusts the determination of the exact time to the judgment of the hegumenos, whether he thinks it more advantageous to increase the age-requirement for the entering and being established in the married life.  But the canon lessens the time defined by Basil the Great, because the Fathers thought that the Church by divine grace had grown stronger since then, and was going on more and more, and that the faithful seemed firmer and more stable for the observance of the divine commandments.  And for the same reason, viz., that the Church was growing better, the sacred canons had lessened the age of deaconesses, and fixed it at forty years, although the Apostle himself orders that no widow is to be chosen into the Church under sixty years of age.