Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XI

Canon XI.

Let not a presbyter be ordained before he is thirty years of age, even though he be in all respects a worthy man, but let him be made to wait.  For our Lord Jesus Christ was baptized and began to teach in his thirtieth year.

Notes.

Ancient Epitome of Canon XI.

Unless he be xxx. years of age none shall be presbyter, even should he be worthy, following the example of the baptism of our Saviour.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. lxxviii., c. iv.

Gratian.

(Ut supra, Nota.)

This is the law, and we do not read that Christ, or John the Baptist, or Ezechiel, or some other of the Prophets prophesied or preached before that age.  But Jeremiah and Daniel we read received the spirit of prophecy before they had arrived even at youth, and David and Solomon are found to have been anointed in their youth, also John the Evangelist, while still a youth, was chosen by the Lord for an Apostle, and we find that with the rest he was sent forth to preach:  Paul also, as we know, while still a young man was called by the Lord, and was sent out to preach.  The Church in like manner, when necessity compels, is wont to ordain some under thirty years of age.

For this reason Pope Zacharias in his Letter to Boniface the Bishop, number vi., which begins “Benedictus Deus” says,

C. v.  In case of necessity presbyters may be ordained at xxv. years of age.

If men thirty years old cannot be found, and necessity so demand, Levites and priests may be ordained from twenty-five years of age upwards.

Van Espen.

The power of dispensing was committed to the bishop, and at length it was so frequently exercised that in the space of one century [i.e. by the end of the xiith century] the law became abrogated, which was brought about by necessity, so that it passed into law that a presbyter could be ordained at twenty-five.  And from this it may appear how true it is that there is no surer way of destroying discipline and abrogating law than the allowing of dispensations and relaxations.  Vide Thomassinus, De Disc. Eccles., Pars. IV., Lib. I., cap. 46.