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

p. 373 Canon XV.

A subdeacon is not to be ordained under twenty years of age.  And if any one in any grade of the priesthood shall have been ordained contrary to the prescribed time let him be deposed.

Notes.

Ancient Epitome of Canon XV.

Those shall be chosen as Subdeacons who are twenty years of age.

This age seems first to have been fixed by the Second Council of Toledo 362 (circa, a.d. 535) in its first canon.

Footnotes

It is curious that so learned a scholar as the late Henry Bradshaw in his article “Subdeacon” in Smith & Cheetham’sDictionary of Christ. Antiq. should give the date of this synod as 447.  Hefele fixes it at 527 or 531.  Baronius, Binius, Labbe, and many others at 531.  A very ancient ms. assigns it to the year 565 of the Spanish era, i.e. 527, and this is the date Cardinal de Aguirre adopts, and is also the one given to the council by the editors of L’Art de Vérifier les dates.