The Mishna in Avos says that one should get married at 18 , is there a reason for this particular age ?
Sorry for all the lack of memory here, but this is from a long time ago:
I once heard a shiur (I think it was from R' Hershel Schachter, but I really am not sure) in which he suggested that one would begin learning Talmud at age 15 (previous clause of source in the question), and would get married when one had reached a degree of completion of his learning. This is in line with the sentiment in Kiddushin 29b that a person should first learn Torah and then get married. The time it takes to learn takes either 3 or 5 years (Chullin 24a), and there are two opinions on when one should get married - either 18 or 20 (Kiddushin 29b). Both agree that one should start learning at age 15, and both agree that one should accomplish their learning before getting married, and the disagreement is how long that takes, either 3 or 5 years, which yields the two ages of 18 and 20.
(This was partially reconstructed based on what I remember and a source provided by @Fred)