I think the reason that boys wait so till they are older already to get married is that boys have a cheyuv of talmud torah as opposed to a girl….when a boy is married already then he has other responsibilities that will take away from his overall learning (I.E. watching kids)….when a bochur waits till he is older already to get married he has a much more solid foundation on which to rely to get him through the times which can be tough for his learning…if he got married early he would fall out of learning altogether…a girl doesn’t have a cheyuv talmud torah (according to me at least) and therefor can get married earlier…the only sevarah that i have ever heard from a reliable talmud chacham for getting married earlier is when your taivah is to starong for you and it is taking away from your learning then you can get married earlier and have “pas be’salo”….but that is only when it is better for your learning.
P.S. to PM: i hope you where kidding when you said that we should get rid of the cherem derabbeinu gershom….he was one of the rishonim and we and all of our gedolay hador combined don’t come up to his toenails…if he said that we shouldn’t do it there is absolutely nothing that should change that.