The Canons of the Council in Trullo 15

15 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 (circa, a.d. 535) in its first canon.

Philip Schaff and Henry Wace (eds.), New York: Christian Literature Publishing Co., 1890-1900. Public domain: English translation first published 1885-1900 (Nicene and Post-Nicene Fathers, Second Series).