The first Mitzva mentioned in the Torah is Peryia veRibyia, the commandment (and God's first blessing!) to get married and bring children to this world (see here).
Two thousand years ago, in Pirke Abot, the Rabbis said that a boy should get married when he reaches 18 years old. They said that God himself 'watches him and waits for him' from 18 to 20 to see him getting married. However, the Rabbis themselves explained that if the boy is busy with his studies and fears that once married he will have to stop studying (veitbabtel min haTorah) he could postpone his marriage (Shulchan Arukh, Eben haEzer 1:3). Some rabbis suggest that marriage should not be postponed beyond the age of 24 years old.
Clearly, according to our Rabbis, it is preferable to get married young. But they themselves acknowledge that there are other elements beyond age to be taken into consideration. For example, the maturity of the boy and the girl, which is essential to have a happy life (=Shalom Bayit) and the possibility to provide for the basic expenses of a family. Maimonides wrote: "Those who are emotionally-balanced (derekh ba'ale hade'a)they first secure a job which enables them to provide for their livelihood, then they get a home, and then, they get married. But those who are emotionally immature (tipeshim), first they get married, then they try to get a place to live, and then they look for a job..." (De'ot 5:11).
We can see that, in the Rabbis' opinion, the younger one gets married, the better. Especially when one is emotionally mature and has the means to live a decent life, marriage should not be postponed! From the other side, many other factors need to be considered in this equation, as the circumstances differ from individual to individual.
