Depending on the level of retardation, the court may see that person as a minor at any age. In order to legally have sex with them, you would have to get consensual permission from their legal guardian. If you marry them I think you automatically become responsible. Kinda off topic but I had a friend whos mom had a stroke and is now not mentally capable of making her own decisions. A POA was issued to his dad he now has the say in what happens in her life.