Depending on where you live, if you had sex with your boyfriend, he might be guilty of statutory rape.
As to why it is such a big deal for teens:
In most places it is believed that until a child reaches a certain age, the child is not really able to give informed consent to sex. In other words, they can't really understand what they are consenting to. Most of us would agree that an 8-year old girl could not give informed consent to sex. That is, even if the 8-year old girl agreed, it would still be wrong, because she wouldn't understand what she was agreeing to.