Why marry at eighteen if one's reasoning isn't great yet?
Although I can't seem to find it now, I recall a rule that says that no one under the age of twenty can sell real property that he's inherited. Why? Because people under the age of twenty are not trusted to make reasoned decisions.
We also find that God does not punish people under the age of twenty. I'm guessing that this is for a similar reason.
Yet the recommended time to marry is at age eighteen. Whom to marry is one of the most important choices one makes. How do we understand that it should be done even before one can sell ancestral property?
(If marriages were wholly arranged, it could perhaps be understandable. But, even in the cultures of arranged marriages, Judaism has a rule that a man must decide to marry his wife.)