When someone is on the sex offender registry it's usually for the persons life meaning you'll be on the sex offender registry for the rest of your life.
However in some cases if you can prove that you're never gonna offend as a sex offender again and have been on the sex offender registry for at least 10 years and have not been in trouble in those 10 years or have never again committed a sexual offense then you can sometimes get removed from the sex offender registry.
You have to go to court before your name can be removed from the sex offender registry and it's not very easy to get removed from the sex offender registry.
Rarely does a sex offender get removed from the sex offender registry especially if the sex offender raped a child because most places consider sex with a child or rape of a child a serious thing which it is.
Even if a minor agrees or consents to sex with an adult it's still wrong and the adult should always say NO.