Parashat במדברNumbers 4:3 –No student left behind
Numbers 12:3 “from thirty years old and upward even until fifty years old, all that enter upon the service, to do work in the tent of meeting ”
Hullin 24a: “25 years old to learn and 30 years old to serve. From here we learn that a student that did not see positive signs in his learning after five years will never see them”
Torah Temimah Colloquial Translation on Note #2:
The Torah specified these five years for learning because these are the last stage. Thus, one who does not see positive signs in learning, during these years, will never see them. Based on this, one can explain why the Tanah of the Mishnah in Ethics of the Fathers (5:21) lists the ages for learning in increments of five years (5 years old for scripture, 10 years old for Mishnah, 15 years old for Gemara. Therefore, if a nine year old student does not see positive signs in his scripture, one is obligated to work with him one more year because there is still hope that the student will improve in scripture. The same applies to all of the stages mentioned in the Mishnah.
See also the comment of on Nachmanides on this midrash in his Torah commentary (Parashat Behaalotecha – Numbers 8:24)
I do not know if this opinion is according to all the Rabbis for I have seen this statement in the Sifrei attributed to Rabbi Natan.
I do not know how to interpret Nachmanides because both the Gemara and the Sifrei that we have quote this learning anonymously.
The Torah Temimah stresses how important it is never to give up on students who are slow learners. Here Torah, here gives the Levites five years to learn the trade, as it were. The Mishnah he cites gives five years for each stage of learning. He stresses that it is most import to work with the student toward the end while there is still hope for the student to progress. This is usually long after most teachers have given up in despair.
Many times, the Torah Temimah tries to resolve contradictory versions in the sources. Here, he admits, that ,based the sources available, he is unable to interpret Nachmanide’s comment.
 Nachmanide’s copy of Sifrei mentions Rabbi Natan. See Torat Chaim note 24 on the commentary of Nachmanides. The Horrowitz edition of Sifrei mentions that there are versions that do not mention Rabbi Natan, implying that some do.