For my son, he 介片 before 3 years old. Actually the age for 介片 may be different for different child, depends on whether your kids is ready or not.
However, I also controlled his last milk. It will only be 5 oz maximum, if he does not drink any soup for dinner. If he drinks soup, it will only be half bowl after dinner and just 3-5 oz milk.
I never wake up my son at midnight. Of course, he needs to vee vee before going to bed.
When kids grows bigger, diaper is difficult to hold all urine as it's too much at a time. Even when my son still needs to wear traning pants at night in the past, the bed always get wet.