Why are 20 year old buchrim still in Mesivta?
As to them being allowed to date, sure why not? In the non-frum world they date in high-school. Of course, they try to teach them about the consequences of having a child then and how it might impact your life. When you look at the world it does seem that having a child in high school, seriously impacts your ability to form a healthy functioning family on secure financial footing (one of those things you take responsibility for when you write a Kesuba).