The first commandment in the Torah is Peru U'Revu, the commandment to "be fruitful and multiply" (Bereshit 1:28).
When does the obligation to fulfill this commandment begin? Does the Mitzva of Peru U'Revu start at 13, when a child becomes Bar Mitzvah, and is obligated in Mitzvot? Or perhaps 18, since the Mishnah tells us that 18 is the age to get married? Or perhaps another age?
If not 13 - why not? (sources please)