Not knowing you have to register something or not having the mental capacity to understand how to register is not a defense to a registration violation according to the Supreme Court of Kansas.
A person required to register in Kansas was convicted of a failure to register offense. He was required to report quarterly, every May, August, November, and February, plus changes.On September 18, he went in to report a change of phone number. He went in again on October 9 to report an address change. But he failed to show up for his registration appointment in November, instead coming in again on December 15th to register. He was arrested for missing the November registration.
The court held that registration is a strict liability offense and there is no intent or mens rea requirement. You can read the opinion here: https://www.kscourts.org/KSCourts/media/KsCourts/Opinions/121014_1.pdf