The problem with that approach is that females have a very alpha attitude especially in our community where family size is something to envy. So you would not be able to hold back that kind of force of nature.
If you told girls that they werent going to be put into the shidduch system until age 21 or 22 you would have 3 Groups.
Group 1 – they would abide by the rules and find something constructive to do with their time (I.E. go to school, work, etc)
Group 2 – would abide by the rules publicly but have their parents/friends working behind the scenes to get them married off so they can have a “head start” on their friends
Group 3 – People who view the rule as an invalid solution and do not consider it binding.
So there you have it, the rule would not have a lasting or widespread effect and to the contrary might make things worse.