I found an English translation of the Mishnah
with conservative commentary. I wanted to see the sources of the Rabbi and Talamidim article. So here is the complete mishnah from the piece quoted in the article:

Avoth 5 Mishnah 21

הוּא הָיָה אוֹמֵר, בֶּן חָמֵשׁ ×©×Ö¸× Ö´×™× לַמִּקְרָא, בֶּן עֶשֶׂר ×œÖ·×žÖ¼Ö´×©×Ö°× Ö¸×”, בֶּן שְׁלשׁ עֶשְׂרֵה לַמִּצְוֹת, בֶּן חֲמֵשׁ עֶשְׂרֵה לַתַּלְמוּד, בֶּן ×©×Ö°×ž×•Ö¹× Ö¶×” עֶשְׂרֵה לַחֻפָּה, בֶּן עֶשְׂרִים לִרְדּוֹף, בֶּן שְׁלשִׁים לַכֹּחַ, בֶּן אַרְבָּעִים ×œÖ·×‘Ö¼Ö´×™× Ö¸×”, בֶּן חֲמִשִּׁים לָעֵצָה, בֶּן שִׁשִּׁים ×œÖ·×–Ö´×§Ö°× Ö¸×”, בֶּן שִׁבְעִים לַשֵּיבָה, בֶּן ×©×Ö°×ž×•Ö¹× Ö´×™× לַגְּבוּרָה, בֶּן תִּשְׁעִים לָשׁוּחַ, בֶּן מֵאָה כְּאִלּוּ מֵת וְעָבַר וּבָטֵל מִן הָעוֹלָם:

1. He used to say:
2. At five years of age the study of Scripture;
3. At ten the study of Mishnah;
4. At thirteen subject to the commandments;
5. At fifteen the study of Talmud;
6. At eighteen the bridal canopy;
7. At twenty for pursuit [of livelihood];
8. At thirty the peak of strength;
9. At forty wisdom;
10. At fifty able to give counsel;
11. At sixty old age;
12. At seventy fullness of years;
13. At eighty the age of “strength”;
14. At ninety a bent body;
15. At one hundred, as good as dead and gone completely out of the world.

The commentators make a note that these ages were probably meant as a guide for parents in teaching their children. In modern times, some the categories no longer apply. For example, it was typical for a man to become a rabbi at 40. These days, rabbis usually start at a much younger age. Also, 90 in our time is not necessarily the same quality of life of 90 in the times just after the temple was destroyed.

I haven’t found much online written by Shmuel Safrai, but I keep finding places where he is quoted. One such article appears quite interesting on the surface. It contends that Jesus was a Pharisee of the School of Hillel and that the Pharisees he blasts were Pharisees of the School of Shammai. Shammai, it seems, was a stern and strict interpretor of the Torah, the opposite of Hillel. However, it seems that Hillel, not Shammai appears to be quite strict in regards to dietary laws. In any case, reading the two of them going back and forth about eggs laid on festivals days (can they be eaten? Eduyoth 4:1), sewing tassles on bedsheets (Eduyoth 4:10), and when divorce is really divorce (Eduyoth 4:7) can help to understand Jesus’ words about straining at gnats and swallowing camels (Matt 23:24).

Personally, it appears to me that Jesus used a form of teaching that was familiar to the Jews in Galilee and Jerusalem. However, he did not follow the structure of it completely by asking “unschooled men” to be his students. He also made clear that although he was a teacher, no one after him was to be called teacher or rabbi (Matt 23:10). As such, when he called the apostles to teach the nations in the Great Commission, it seems to me that they were to teach that Jesus is the teacher and make students of Jesus. To the method, however, it appears that there is some flexibility. Paul used the Hall of Tyrannus for teaching Jews and Gentiles, but appears to have had a small group that followed him personally (Acts 19:9-10), as well as teaching in synagouges (Acts 17:2 and many other places). As such, I believe the methods of teaching and training described in the Bible are descriptive, not proscriptive. Paul was diverse in technique, I see no reason why we cannot be diverse as well.