“This is not very feasible. How will you convince a girl of 19 to not date in the midst of a shidduch crisis?”
Perhaps we could convince them to work for a few years before going onto the market, and besides then they’d have some money to start off with too.
“Who will decree this and be accepted?”
Now we get to the hard part. Maybe the Agudah? Probably still wouldn’t have much of an effect, I know. The only way to get people to listen is to do something that bites – like, get a whole bunch of Rabbonim to sign that they won’t attend any chasunos where the girl is under 20/21. And maybe convince some halls/caterers not to service these weddings, too.