ujm:
Go ahead. Make sure that every bochur is married by 20. But then you should follow the suggestion of Rav Henkin ZT”L. He recommended that when the rav arranges the writing and signing of the kesubah, he should attend to the gett. He probably said that tongue in cheek. But your proposal makes it necessary.
The age for marriage is determined by way too many other factors. And your making date of birth the only one to matter, you are essentially doing publicity for toanim and batei din, and for lawyers and courts. I caould also add in therapists and askanim.
Your statement of “maximum age of 20” is not pshat in the Gemora. So the Roshei Yeshivos who had talmidim learning well past that age were מסייעים לדבר עבירה? If that’s your message, I will not participate in any further discussion.