Avos 5:20 - 5:21

"Be brazen like the leopard": This leopard is born from a wild boar and a lioness. As when it is the season for lions to be in heat, the lioness inserts its head into the brush of the forest and moans and summons the male, but [sometimes] the boar hears its voice and copulates [with] it and a leopard comes out from [the combination of] the two of them. And because it a bastard, it is brazen-faced, even though it does not have so much courage. You too, be brazen and do not be ashamed to ask your teacher what you did not understand, like that which we have learned, "one that is ashamed will not learn."

"light like the eagle": to pursue after your studies. And you will not get exhausted, as it is written (Isaiah 40:31), "they shall raise wings like eagles, they will run and not be exhausted."

"swift like the deer": to run towards the commandments.

"and mighty like the lion": to conquer your impulse from [the doing of] sins.

"brazen-faced": Since brazenness is noticeable in the face, as it is written (Proverbs 21:29), "The brazenness of an evil man is in his face": therefore he is called brazen-faced.

"that Your city be rebuilt, speedily and in our days": Meaning to say, in the same way that You have graced us with this trait - since it is a sign [of people being] of the seed of Abraham that they are [easily] ashamed, merciful and doers of acts of loving-kindness (Yevamot 79a) - so may it be Your will that Your city be rebuilt, etc.

"Five years [is the age] for [the study of] Scripture": We learn it from orlah (the prohibition to eat fruit from a new tree), as it is written (Leviticus 19:23-25), "three years will they be orlah, etc. And in the fourth year all of its fruit will be holy praises" - that his father should teach him the shape of the letters and recognition of the vowels [in the child's fourth year]. "And in the fifth year you shall eat its fruit; [so that] you will add to your produce" - from here and onward, stuff him like an ox.

"Ten [is the age] for [the study of] Mishnah": As he studies Scripture five years, and Mishnah for five years, and Talmud for five years, as the Master said (Chullin 24a), "Any student who has not seen a positive sign in his study for five years will not see one further [than that]" - as it is written (Numbers 8:24), "This is [the rule] among the Levites; from twenty-five years and above, he should come to be of the service," that he should come to learn the laws of the service for five years and at thirty, he should serve.

"Thirteen [is the age] for [observing] commandments": As is written (Numbers 5:6), "a man or a women who have done from all of the sins of man." And regarding Shekhem, it is written (Genesis 34:25), "and the two sons of Yaakov, Shimon and Levi, took - each man - his sword." And at that time, Levi was thirteen years old, and [still, the verse] calls him a man.

"Eighteen [is the age] for the [wedding] canopy": [The word,] man is written nineteen times in Parshat Bereshit (the beginning of Genesis) from, "And God said, 'let us make man'" (Genesis 1:26), until "And the Lord God built the side" (Genesis 2:22, when Eve was created). One [use of the word, man] is for itself [such that] eighteen remain to be expounded [in this way].

"Twenty [is the age] for pursuit": of his sustenance. After he has studied Scripture, Mishnah and Talmud, and married a woman and produced children, he must seek out sustenance. A different explanation: Twenty [is the age] of his being pursued from the Heavens and to punish him for his deeds; as the court Above does not punish [someone] less than twenty years old.

"Thirty [is the age] for [full] strength": As the Levites would put up and take down the Tabernacle and load the carts and carry on their shoulders from [the age] of thirty and above.

"Forty [is the age] for understanding": Since after forty years that Israel was in the wilderness, Moshe said to them (Deuteronomy 29:3), "'And the Lord did not give you a heart to know and eyes to see and ears to hear until this day.'"

"Fifty [is the age] for [giving] counsel": As it is stated with the Levites (Numbers 8:25), "And from fifty years, he shall return from service and not serve more; and serve his brothers, etc." - and what is the service? That he give them counsel.

"Sixty [is the age] for mature age": As is written (Job 5:26), "You will come in your old age (bekhelach) to the grave." The numerical equivalent of [the letters in] bekhelach is sixty.

"Seventy [is the age] for a hoary head:" As is written about David (I Chronicles 29:28), "And he died with a good hoary head"; and the days of his life were seventy years.

"Eighty [is the age] for [superadded] strength (gevurot)": As is written (Psalms 90:10), "and if with great strength (gevurot), eighty years."

"Ninety [is the age] for [a] bending [stature]": He walks stooping and bent over. And some say, it is an expression [similar to its usage in Proverbs 22:14], "A deep pit."