very difficult to enforce. even your statutory rape example - 1/3rd of young girls have sex by age 16, 2/3rds by age 18. That's alot of guys you need to put in jail.
If DHS f/k/a the INS finds illegal workers at a Tyson Poultry Plant, then only one person goes to jail and that person's name is Tyson. You can bet if Tyson has to spend a year in jail, his company will never, ever hire an illegal again.