Although at the age of thirteen a male becomes obligated to fulfill all the mitzvot, the Sages say that the ideal age for a male to get married is eighteen, and no later than twenty. This delay is because he must prepare himself for the challenges of raising a family in two areas. The first is mastery of the basics of the Torah to mold his worldview and so that he knows how to act in accordance with halakha. This is what the Sages mean when they say, “Five years old is the age to begin studying Scripture; ten for Mishna; thirteen for the obligation of the commandments; fifteen for the study of Talmud; eighteen for marriage” (Avot 5:21). Elsewhere the Sages say that studying Torah should come before marriage, because if one marries first, the burden of supporting a family is likely to impede his Torah study (Kiddushin 29b). Similarly, Shulḥan Arukh rules, “A man should first learn Torah and then marry. For if he marries first, he will not be able to devote himself to Torah study, as he will have a millstone around his neck” (YD 246:2).
The second type of preparation is learning how to earn a living. It used to be that while a young man was studying the basics of the Torah, he devoted part of his day to working with his father and thus learning a trade that could earn him a living, build a home, and save up to invest in the furtherance of his career. The idea that one must have a means of support before getting married is derived by the Sages from the order of the verses in the Torah: “Who has built a new house…planted a vineyard…betrothed a woman…” (Devarim 20:5-7). The Talmud states:
The Torah teaches us the proper way to go about things. First, one should build a house, then he should plant a vineyard, and only then should he get married. King Shlomo, too, wisely declared, “Put your external affairs in order; get ready what you have in the field, then build yourself a home” (Mishlei 24:27). “Put your external affairs in order” refers to the house; “get ready what you have in the field” refers to the vineyard; “then build yourself a home” refers to getting married. (Sota 44a)
Rambam similarly writes:
It is the way of intelligent people to ensure that first they learn a trade with which they can support themselves, then buy a home, and afterwards they marry…. But fools marry first; then, if he can, he buys a house, and then, at the end, he will try to learn a trade or will live off charity… (MT, Laws of Dispositions 5:11)
Therefore, the Sages instruct men to postpone marriage until the age of eighteen. At the same time, they also warn against delaying marriage beyond the age of twenty, saying (Kiddushin 29b), “Until a man turns twenty, God sits and waits for him to get married. If he reaches the age of twenty and is not yet married, God says, ‘Let his bones swell up!’” In other words, he is cursed for not fulfilling the mitzva of procreation. The Sages also comment on the verse, “A time for giving birth and a time for dying” (Kohelet 3:2): “From the time of a person’s birth until he turns twenty, God awaits his marriage. If he reaches the age of twenty and has not yet married, God says to him, ‘There was a time for you to give birth, but you were not interested; now it is only a time to die’” (Kohelet Rabba 3:3).
Furthermore, the Sages state, “If a man does not marry by the age of twenty, he spends his whole life thinking sinful thoughts” (Kiddushin 29b), for as long as a man who has not yet reached that age knows that when the time comes he will marry and love his wife as himself, then even if he sometimes entertains sinful thoughts and transgresses by masturbating, he knows that it is improper and that when he is married he will reserve all his desire for his wife. However, when bachelorhood lasts too long, he despairs of mastering his desire, surrenders to it, and gets used to gratifying himself sinfully. Then, even when he marries and is faithful to his wife, it will be difficult for him to avoid sinful thoughts, because they will have become a part of him. Only if he repents sincerely out of love will he be able to correct this (above, 4:2, n. 2).
Some people manage to marry early, either because they had help from their parents or because they were exceptionally talented, and this is praiseworthy. As R. Ḥisda said of himself, his preeminence was not because he was more talented or more righteous than his colleagues, but because he was able to marry at sixteen and thus learn Torah in purity without sexual temptation. He added that had he gotten married at fourteen, he would have been so immune to the evil inclination that he would have been able to taunt Satan without fear of being tempted to sin (Kiddushin 29b-30a).
The beit din compels a man to marry in order to fulfill the mitzva of procreation, even though the mitzva of procreation remains constant throughout life and one is exempt after fulfilling it, so one who is lazy about it has postponed the mitzva but not completely disregarded it. Nevertheless, because he is commanded, he is still disregarding the fulfillment of a mitzva…. [Therefore,] if a man takes an oath that he will not get married until after he turns twenty, even if he specifies that he will delay it for only a year, he has taken an oath to disregard a mitzva, and the oath therefore does not take effect. (Responsa Maharit YD §47; likewise Shiyarei Knesset Ha-gedola [YD 236, Hagahot Tur 44] and Yafeh La-lev vol. 4, EH 1:12)
However, according to Rashba, one who swears that he will not get married until he is over the age of twenty is not considered to have taken an oath to disregard a positive commandment, since he can fulfill the mitzva later (Responsa Rashba 4:91). Ḥikrei Lev (EH §1) agrees, since it is possible to fulfill the mitzva at a time later than the ideal. As long as it is possible for him to keep his oath and still fulfill the mitzva, he is not considered to have sworn to disregard a mitzva. Nevertheless, Ḥikrei Lev adds, “He is punished by heaven for the time during which he disregards a positive commandment; furthermore, he could die before fulfilling the mitzva, in which case he will in fact have disregarded the positive commandment entirely.” We can also posit that Rashba’s leniency does not mean he is denying the rabbinic requirement to marry by the age of twenty. Rather, Rashba maintains that an oath that negates a rabbinic mitzva takes effect (Responsa Rashba 1:614).
It is important to add that timing plays a significant role in fulfilling the mitzva of procreation, since the goal of the mitzva is to multiply and fill the world with people (Bereishit 1:28 and 9:7). Multiplying is dependent on two factors – how many children a couple would like to have, and when they begin having them. The younger that people are when they begin having children, the faster the next generation can begin to multiply. This is why the Sages prohibited getting married on Ḥol Ha-mo’ed. If it were permitted, people might delay marriage by a few months in order to get married then, and that would negatively impact procreation (Ḥagiga 8b; SA 546:1-2; Peninei Halakha: Mo’adim 10:4). The Sages also said, “Yehoshua was punished only because he prevented the Jewish people from fulfilling the mitzva of procreation for one night” (Eruvin 63b).
Other critical reasons to insist on marriage by the age of twenty are the fulfillment of the mitzva of ona and the prevention of sinful thoughts, as explained in Kiddushin 29b (see above). Rambam codifies: “Likewise, it is rabbinically mandated that a man not remain single, so that he will not have sinful thoughts” (MT, Laws of Marriage 15:16). Ritva (Yevamot 61b); Hasagot Ha-Ra’avad and Nimukei Yosef (Yevamot 19b in Rif pages); Levush 1:8; and AHS 1:7 all write similarly. See 4:7-8 above.