How retarded.... the current rules limit to 4 years older for the prospective spouse, there is court oversight, counseling, and the younger must be a minimum of 17 years old. Sounds reasonable and fair to me.
The only issue I can see is if the younger is living at the parent's home and doesn't want to leave or the parents are penalized up till the kid turns 18, 21, 25 for various things like insurance, grants, scholarships, et cetera.
They should probably start from a legal emancipation basis and not just grant those for less socially prevalent hardships at home (ie "my parents are fuckwits and I'm raising myself on my own income" sort of thing) but for exemplary adulthood status before 18.