I saw this post and felt I had to respond. This is an amazing idea.
1) we are in a financial crises and many people are out of work. Many parents cannot support a kollel couple as many years as the couple might like. If they wait until she is 20 then there will be time for her to start saving money so the parents do not have to, as well as give more time for the parents to save.
2) girls will have the opportunity to go for shana bes and not have to worry about this “ruining her chances”. girls mature a lot when they go away for seminary, especially shana bes. Many girls I have met grew so much their shana bes year that I was glad they went before getting into the parsha.
3) Girls can have the opportunity to (gasp) get a degree. We see the necessity for the woman to work and not everyone can be a teacher. Mathematically this cannot work. Someone will be out of a job because if every mother sent their kids to school, and taught at the school, then there must be incredibly small classes or many mothers without jobs. And there arent that many secretary jobs out there in schools. There are kosher ways, in good environments, to get educated. Thats for a different post tho.
4) closing the age gap, as mentioned previously.
5)girls can mature and grow up without having the pressure that they are “old maids”. They can learn the skills needed to run a house that they were not able to learn while in high school because they were busy with school work, activities, productions ect.