I saw a post on one of the news group.  Here's the question.

Why is it that if a minor 12 years or older can be sentenced as an adult in a crime, but when a minor 12 years or older has sex with someone over the age of 18 its a crime for older person, but not for the minor???

One is a perpetrator, the other a victim.  Believe under the law, a minor is presumed to be unable to give consent to being victimized.

What you have posed is a legal question. And the answer given already is fairly accurate. But there's a flaw in your question.
The law states that a minor, i.e. one under age 18, cannot legally give consent. Minor's can't buy or register motor vehicles, enter a binding contract, and most certainly, consent to a sex act. As far as your question goes, only a few states allow a minor at age 12 to be sentenced as an adult.