Why did the Benedictines stop educating young boys inside their monasteries?

William of Tocco, O.P., mentions in ch. 5 of his biography of St. Thomas Aquinas (pp. 33-4) that: >once he [St. Thomas] had reached the age of five, they sent him to the [Benedictine] monastery of Monte Cassino in the arms of his nurse. […] Very soon, this child began to receive his education in the monastery under the diligent instruction of a master to whom he showed clear signs of his future advancement. Dom Delatte's commentary on *St. Benedict's Rule* ch. 59 (p. 406 ) says: > children [were] received into the monastery temporarily as *alumni*, to be educated there in contrast to "children [who were] given permanently and devoted to the religious life", which was later forbidden by the Council of Trent session 25, ch. 15: "Profession Shall Not Be Made Except After One Year’s Probation and on the Completion of the Sixteenth Year". Why did the Benedictines stop admitting such young boys to their monasteries for educating them?