In Kovetz Shiurim (Vol. 2, siman 19) R’ Elchonon asks: How can one push off getting married because of the rule that ‘one who is busy with one mitzvah is exempt from another’, surely, peru u’revu is a mitzvah which can’t be carried out by someone else (אי אפשר לקיימה על ידי אחרים), and it is clear from the Gemara in many places that by a mitzvah which can’t be carried out by someone else, we don’t apply the rule of ‘one who is busy with one mitzvah is exempt from another’?
R' Elchonon answers: Since one is only delaying the mitzvah, but not pushing it off entirely, and one can carry it out himself at a later date, לא פקעה ממנו מצות ת"ת – the obligation to learn Torah doesn’t leave him, and since the obligation of talmud Torah is upon him, he is exempt from other mitzvos.
The Shulchan Aruch HaRav (Hilchos Talmud Torah, Perek 3, sif 1 and Kuntros Achron, Perek 3, he’orah 1) answers: The truth is, the mitzvah of peru u’revu is like all mitzvos, that if it can’t be carried out by others, it overrides the mitzvah of learning Torah, however, this is only if one can interrupt for a short time and then return to his learning. However, the yoke of earning a parnosah is a constant bitul Torah, and it is possible that it may cause one to forget lots of his Torah as he won’t have time to review it, and the mitzvah of yedias haTorah [knowing Torah] isn’t pushed aside for the mitzvah of peru u’revu, even though it can’t be carried out through someone else. Therefore, he concludes: If one has a good memory and is able to learn and remember the Torah, he shouldn’t get married until he finishes the entire Torah She’ba’al Peh [Oral Torah], if, however, one isn’t able to do this, then he may get married before doing so.
He adds: In the times of Chazal, they learnt with young children, 5 years Mishnah (from 10 – 15), five years Talmud (from age 15 – 20) and they got married at age 20. Sometimes they got married already from age 18 as even after getting married one is normally able to learn 2 – 3 years without any great distractions, before one has lots of children. He then quotes Rashi and the Bartenura (Avos, end of Perek 5) who both explain: בן עשרים לרדוף - age 20 is the age for running, to mean at age 20 one must run after finding a parnosah.
