Does the practice of 20+ year-old single men not in yeshiva violate halacha?
In principle, the age for marriage is 18, as mandated by the Mishnah (Avos 5:21). By that age, a person is expected to have moulded his personality and studied sufficiently to enable his marriage. However, a person is permitted to delay his wedding for the sake of Torah study (Shulchan Aruch, Even Ha’ezer 1:3), even past the age of 20, which the Gemara cites as a “deadline” for marriage (Kiddushin 29b). Some permit the postponement of marriage until the age of 24, and no further (Yam Shem Shlomo, Kiddushin).
It thus emerges that past the age of 20 there is no halachic permission for a person to remain single, unless he is immersed in Torah study. However, the foundation for even marriage is shalom bayis, and harmony in the home cannot be achieved unless the partners to it are mature enough to enter the deep relationship of marriage. Questions of emotional maturity, of the ability to financially sustain the home, and, of course, of finding the right bride, therefore play an important role in determining the age of marriage.