"At twenty years of age, the will reigns; at thirty, the wit; and at forty, the judgment."
-Benjamin Franklin
וַיְהִי בַּיָּמִים הָהֵם וַיִּגְדַּל משֶׁה, בֶּן עֶשְׂרִים שָׁנָה הָיָה משֶׁה בְּאוֹתָהּ שָׁעָה, וְיֵשׁ אוֹמְרִים בֶּן אַרְבָּעִים. וַיִּגְדַּל משֶׁה, וְכִי אֵין הַכֹּל גְּדֵלִים, אֶלָּא לוֹמַר לָךְ שֶׁהָיָה גָדֵל שֶׁלֹא כְּדֶרֶךְ כָּל הָעוֹלָם...
And it came to pass in those days, and Moses grew up (Exodus 2:11). Moses was 20 years old at the time, and some say 40 years old.
"When Moses was grown up" - and does not everyone grow up? Rather, this tells you that he grew up [in a manner] unlike the whole world...
Acts 7:22-23
22] Moses was educated [in] all the wisdom of the Egyptians and was powerful in his words and deeds.
23] When he was forty years old, he decided to visit his kinsfolk, the Israelites.
(כא) הוּא הָיָה אוֹמֵר, בֶּן חָמֵשׁ שָׁנִים לַמִּקְרָא, בֶּן עֶשֶׂר לַמִּשְׁנָה, בֶּן שְׁלשׁ עֶשְׂרֵה לַמִּצְוֹת, בֶּן חֲמֵשׁ עֶשְׂרֵה לַתַּלְמוּד, בֶּן שְׁמֹנֶה עֶשְׂרֵה לַחֻפָּה, בֶּן עֶשְׂרִים לִרְדֹּף, בֶּן שְׁלשִׁים לַכֹּחַ, בֶּן אַרְבָּעִים לַבִּינָה, בֶּן חֲמִשִּׁים לָעֵצָה, בֶּן שִׁשִּׁים לַזִּקְנָה, בֶּן שִׁבְעִים לַשֵּׂיבָה, בֶּן שְׁמֹנִים לַגְּבוּרָה, בֶּן תִּשְׁעִים לָשׁוּחַ, בֶּן מֵאָה כְּאִלּוּ מֵת וְעָבַר וּבָטֵל מִן הָעוֹלָם:
(21) [Judah ben Tama] used to say: Five years of age for Scripture; Ten for Mishnah; Thirteen for commandments; Fifteen for Talmud; Eighteen the bridal canopy; Twenty for pursuit [of livelihood]; Thirty for strength; Forty for wisdom; Fifty for counsel; Sixty for old age; Seventy for fullness of years; Eighty for might; Ninety for a bent body; One hundred, as good as dead and gone completely out of the world.
...It is stated: “And I have led you forty years in the wilderness” (Deuteronomy 29:4). And it is written: “But the Eternal has not given you a heart to know, and eyes to see, and ears to hear, until this day” (Deuteronomy 29:3). Rabba said: Conclude from here that a person does not understand the opinion of their teacher until forty years.
מה היה תחלתו של רבי עקיבא. אמרו בן ארבעים שנה היה ולא שנה כלום...
What were the origins of Rabbi Akiva? They say that he was forty years old and had still not learned anything...
(ג) וּבֶן אַרְבָּעִים שָׁנָה הִכִּיר אַבְרָהָם אֶת בּוֹרְאוֹ...
(3) And, when Abraham was forty years old he recognized his Creator...
...תניא כל שנותיו של רבי יוחנן בן זכאי מאה ועשרים שנה ארבעים שנה עסק בפרקמטיא מ' שנה למד ארבעים שנה לימד.
It was taught: All the years of Rabbi Yochanan ben Zakkai were 120 years. For forty years he dealt in business, for forty years he studied, and forty years he taught.
It is written in the Torah: “And he shall bathe all his flesh in the water” (Leviticus 15:16), and the Sages derived that nothing should intervene between his flesh and the water. The definite article in the phrase “in the water” , i.e., specifically in the water of a ritual bath. The phrase “all his flesh” indicates that it must be in water into which all of his body can enter. And how much water is that? It is a cubit by a cubit by the height of three cubits. And the Sages calculated that the waters of a ritual bath measure forty se’a.
Forty, the Number from the Jewish Encyclopedia
In the Bible, next to the number seven, the number forty occurs most frequently. In Talmudical literature it is often met with, in many instances having been apparently used as a round number or as a concrete and definite expression in place of the abstract and indefinite "many" or "some," and hence becoming a symbolical number. As regards the period of forty years, the Jews seem to have shared with other peoples, especially the Greeks, the notion that the fortieth year was the height or acme of man's life; and from this fact forty years came to represent a generation (compare Nöldeke, "Untersuchungen zur Kritik des Alten Testaments," p. 188).
Fortysomething by Susan Handelman
Forty, then, represents both the completion of the previous level, and leaving it behind, nullifying it (...the flood lasting forty days), and forty also represents the inauguration and structuring of a new existence (the Torah given in forty days, the development of the fetus, and so forth).
The moment of emptiness contains the seeds of the ascent to a higher level. Like the moon, to which the Jewish people are compared, the cycle of waning is followed by a waxing. Jewish history has had many moments of emptiness, darkness and loss, but just as the moon is renewed monthly after its seeming disappearance, the Jewish people are ever renewed, ever reborn.
In the Jewish view, then, getting older is also getting newer. And turning forty is indeed growing up — an ascent to a higher level. The popular saying that life begins at forty is right. Forty signifies not a period of decline, nor should it be cause for regret. It is, instead, both a completion and a new beginning, a retrospective understanding and a prospective passage to a higher level, an emptiness in the middle and the foundation of an entirely new existence.