For just dating, no. That's not illegal. In most places, it would not be illegal if they were to have sex too, because in most places the age of consent is 16. However in some places where the age of consent is higher then the 21 year old could go to jail because it would be illegal
Well technically no. Only if you guys have sex then he can go to jail. No, there are no laws about dating. There are laws about sexual contact, and in most places the age of consent is 16, though higher in a few places.