§1. Only a priest who has completed thirty-Five years of age and has not already been elected, appointed, or presented for the same vacant see can be designated validly to the function of diocesan administrator.
§2. A priest who is outstanding in doctrine and prudence is to be elected as diocesan administrator.
§3. If the conditions previously mentioned in §1 have been neglected, the metropolitan or, if the metropolitan church itself is vacant, the suffragan bishop senior in promotion, after he has ascertained the truth of the matter, is to designate an administrator in his place. The acts of the one who was elected contrary to the prescripts of §1, however, are null by the law itself.
Source: Code of Canon Law (Vatican.va)