Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Synods of Sardica, Carthage, Constantinople, and Carthage Under St. Cyprian, Which Canons Were Received by the Council in Trullo and Ratified by II. Nice.: Canon CXXVI

p. 503 Canon CXXVI.  (Greek cxxvii.)

That Virgins, even when minors, should be given the veil.

Item, it seemed good that whatever bishop, by the necessity of the dangers of virginal purity, when either a powerful suitor or some ravisher is feared, or if she shall be pricked with some scruple of death that she might die unveiled, at the demand either of her parents or of those to whose care she has been entrusted, shall give the veil to a virgin, or shall have given it while she was under twenty-five years of age, the council which has appointed that number of years shall not oppose him.

Notes.

Ancient Epitome of Canon CXXVI.

Whosoever has veiled or shall veil a virgin before she is twenty-five years of age (that is give her the habit, or clothe her), being forced thereto on account of a powerful lover, or a ravisher, or deadly disease, provided those who have the charge of her so exhort, shall receive no damage from the synod concerning that age.

This is Canon xviij. of Carthage, a.d. 418.  The reference to a former canon is to Canon j. of the second series of the canons of the Synod of Hippo in a.d. 393.