Has the law on banning the entry of young men from Ukraine already been adopted and from what age?

No, the Law you mentioned has not been adopted, there is not even such a bill.

According to Cabinet Resolution No. 57, during martial law restrictions on border crossing apply to citizens of Ukraine from 18 to 60 years old. But there is a list of exceptions for this category as well.

You can read our article - https://ua-lawyer.com/uk/articles/3/khto-z-cholovikiv-maie-pravo-viyizdu-za-kordon /.