Traditionally statutory rape was sex between a minor and an adult. It was considered rape because by definition a minor lacks the capacity to consent. Today some states have laws against statutory rape while others simply define the minimum age requirements in their jurisdiction. West Virginia for example calls sexual relations between someone less than 16 and someone more than 16 statutory rape. CA, on the other hand does not use that language but criminalizes sexual relations between minors and adults, the dividing line being 18 years of age. Such acts can be punished either as misdemeanors or felonies. In CA the difference in age counts as well. Someone 21 or older having sex with someone 16 or younger brings stronger penalties than the simple 18 year rule.
Her Parents Don't Care
Many Avvo questions on this subject assume that if it is ok by the parents it is not illegal. Nothing could be further from the truth. If the child lacks the capacity to give consent the parents cannot supply it for her/him.
Sex Between Children
In many states sex between children will violate the state's penal laws even though both participants are children and entitled to the protection of the law. Thus a 16 year old boy who is having sex with a 14 year old girl can be punished in CA for a misdemeanor violation of the law. The courts recognize that young people may be sexually active but no court has been so advanced as to legitimize these relations.
Although you may not have thought you violated your state's law in having sex with a younger person there are often statutes besides those thought of as statutory rape that may make the behavior criminal. WA, for example, has a law against committing immoral acts with a minor. Watch out.