Age 40 in Halacha and Divrei Chazal:

The age of 40 contains unique significance in both Halachic literature and Chazal, as we will list below.

1. Ben Arbaim Lebina-Age of Binah-Understanding: At the age of 40 one reaches the age of understanding.[1] This means that he is now able to understand the depth of teachings of Torah, and to novelize many new laws and interpretations of verses.[2] This is because at this age a person reaches mental and intellectual maturity.[3]
2. Morei Horah according to all opinions:[4] The Talmud[5] states that at age 40 one reaches the age of Horah, in which he can Pasken. Now, it is debated amongst the Rishonim and Poskim as to the intent of this Talmudic statement. Some Poskim[6] understand it to mean that one may not Pasken until age 40. Other Poskim[7] understand it as discussing one who is able to Pasken but refuses to do so, that until age 40 he may avoid Paskening, although may Pasken if he so chooses, and past age 40 he may not refuse to Pasken. Practically, the Sephardim[8] all rule like the latter approach, and hence there is no issue for a qualified Sephardi Posek to Pasken even prior to age 40. However, the Ashkenazim[9] took the former approach and ruled that one should not Pasken until age 40. Nonetheless, even according to this approach there are various leniencies suggested, and practically the widespread custom of even Ashkenazim is to permit Paskening prior to age 40 despite the ruling of the Rama.[10] Nonetheless, some community guidelines require that an elected Rav be at least 40 years old to be eligible, in light of the above original Talmudic statement and ruling of Rama.
3. Learning Kabbalah:[11] [In previous times,] there was a limitation against studying certain Kabbalistic works until age 40. From age 40 and onwards this became permitted.
4. Publishing Sefarim:[12] In previous times, there were communities which made institution that one may not publish any Sefer in Halacha or Gemara until age 40.
5. Drinking versus eating:[13] Until age 40 eating is more beneficial for the body than drinking. From age 40, drinking is more beneficial for the body than eating.
6. Gezeiras Hamashkeh:[14] The known “Gezeira” of the Rebbe to not drink more than a Revius of Mashkeh [alcohol] applies only until age 40. Although even after age 40 they must diminish in their drinking.
7. Significance of the number 40 in Halacha and Chazal:[15] The sages[16] state that one does not reach an age of understanding of his teacher until age 40. Likewise, the sages[17] state that for 40 days before a child is born they declare in heaven as to whom he will be married to. Likewise, we find that the minimum measurement for a Mikveh is 40 Seah.[18] According to some opinions[19], Avraham recognized his creator at age 40.

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[1] Pirkei Avos 5:21

[2] Admur Hilchos Talmud Torah 1:1; Rashi Avos ibid

[3] See Likkutei Sichos Vol. 20 Noach p. 19

[4] See Likkutei Sichos Vol. 34 Parshas Tavo Sicha 3 [p. 160] in length

[5] Avoda Zara 19a; Sotah 22b

[6] Rama 242:31; Beis Yosef 242 in name of Ran; Rashi Avos ibid; Semak

[7] Michaber C.M. 7:3; Beis Yosef 242; Avkas Rochel 201 p. 361; Kesef Mishneh and Lechem Mishneh on Rambam ibid; Rambam Talmud Torah 5:4; Birkeiy Yosef C.M. 10:5

[8] Michaber ibid

[9] Rama ibid

[10] Rama 242:31 [permits only if no greater Rav in city]; Pischeiy Teshuvah 242:16 in name of Shevus Yaakov 1:140 explains that only Issur Viheter is forbidden while Dinei Mamonus is permitted; Aruch Hashulchan 242:57; See Radbaz 6:2147; Maharashdam C.M. 1; Lechem Mishneh ibid; Divrei Halacha 242:13; Hilchos Talmud Torah of Rav Ashkenazi 2 p. 433; Shevet Hakehasi 1:263; Pesakim Veteshuvos 242:42; See Toras Menachem 5748 2:39

[11] See Shach 242:6

[12] Teshuvah Meahavah in Hosafos on 242:31; See however Yabia Omer 4 C.M. 1

[13] Shabbos 152a

[14] See Sichas Parshas Shemini 1963; Balak 1964; Shmini 1964; Yud Beis Tamuz 1965; Noach 1927; Naso 1968; Shavuos 1968; Dvarim 1980; 13th Nissan 1982; Ki Sisa 1984; Balak 1984; Mikeitz 1985; 12th Teves 1987; Vayakhel 1988; Ki Seitzei 1988; Hisvadyos 1991 Vol. 4 p. 298; See Otzer Minhagei Chabad ibid and Adar 211-214; Kuntrus of “Gezeiras Hamashkeh Bepurim” by Rav Nachmonson;

[15] See Sefer Haerechin Chabad Osiyos Hatorah Mem p. 164

[16] Avoda Zara 5b

[17] Sotah 2a; Sanhedrin 22a

[18] Mishneh Mikvaos

[19] Rambam Avoda Zara 1