According to a precept it is forbidden to integrate a person under 20 years of age into the Sangha. Is that correct? So are the young monks, not monks, but something different?
According to The Bhikkhus' Rules A Guide for Laypeople
Becoming a Novice
The first part of the ordination procedure for bhikkhus is known as the Going Forth into Homelessness (pabbajjaa). If it finishes with just that — without going on to the Questioning of the candidate and the Acceptance of him by all the gathered bhikkhus into the Bhikkhu Sa"ngha — the candidate is known as a saama.nera or novice. This is usually the case when the candidate is less than the twenty years of age necessary to become a bhikkhu. A very young boy is not allowed to become a novice either, but the minimum age will vary according to place.
 "One under 15 years of age, unless he can scare crows (i.e., is mature) should not be given the pabbajaa for becoming a saama.nera (Vin.I,79). After receiving their parent's consent (Vin.I,83), they were to shave their head and beard, put on the ochre robe and, paying respects to the bhikkhu, receive the Three Refuges and the Ten Training Precepts." (HS ch.19)
See the Wikipedia article Śrāmaṇera for a description of Buddhist novices.
Housholder Konrad, interested,
Age requirements. An applicant for the Going-forth must be at least fifteen years old or, if not yet fifteen, "capable of chasing crows away." According to the Commentary, this means that, while holding a clod of earth in one hand, he can chase crows away from food placed in front of him while he is eating it with his other hand.
An applicant for full Acceptance must be at least twenty years old, counting from the time his consciousness first arose at conception in his mother's womb. As this is difficult — if not impossible — to date with any accuracy, the usual practice in calculating a person's age is to add six months to the number of years since his birth, to allow for his having been born prematurely. As the Commentary notes, a baby born after seven months in the womb may survive, but one born after only six months won't. Pc 65 states that if an applicant less than twenty years old receives full Acceptance, he does not count as a bhikkhu; the Commentary says that he remains a novice. Any bhikkhu who acts as his preceptor, knowing that he is too young to be accepted, incurs a pācittiya; any other bhikkhus in the assembly performing the ordination who also know the applicant's age incur a dukkaṭa.
There are certain other things to be regarded proper, of both, one how gives ordination and one who desires, for one not fit for full ordination because of age.
Much on this is found in the Mahavagga of the Vinaya, worthy to know the cases to understand the spirits and limits in regard of going forth under the Tripple Gems better.
(Note that this gift of Dhamma is not dedicated for trade, exchange, stacks or entertainment but as a means to make merits toward release from this wheel)