Are people younger than 20 forbidden to ordain? 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[16] of the ordination[17] 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&quot;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.[18]

Footnote:

[18] &quot;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.&quot; (HS ch.19)

See the Wikipedia article Śrāmaṇera for a description of Buddhist novices.