Council of Trent: Session XXV, Regulars and Nuns, Chapter XV

CHAPTER XV. Profession shall not be made except after a year 's probation, and at the age of sixteen years completed.

In no religious order whatever, shall the profession, whether of men or women, be made before the age of sixteen years is completed; nor shall any one be admitted to profession, who has been less than a year under probation from the time of taking the habit. And any profession made sooner than this shall be null; and shall not superinduce any obligation to the observance of any rule, or of any religious body, or order; or entail any other effects whatsoever.

Translation: J. Waterworth, The Canons and Decrees of the Council of Trent (1848), Hanover Historical Texts Project transcription (1995).